C++ to Java

Java supports the usual forms of if statements:

if (x > 0) {
    System.out.println("x is positive");
}

if (x % 2 == 0) {
    System.out.println("x is even");
} else {
    System.out.println("x is odd");
}

if (x > 0) {
    System.out.println("x is positive");
} else if (x < 0) {
    System.out.println("x is negative");
} else {
    System.out.println("x is zero");
}

if (x == 0) {
    System.out.println("x is zero");
} else {
    if (x > 0) {
        System.out.println("x is positive");
    } else {
        System.out.println("x is negative");
    }
}

The braces are optional (but recommended) for branches that have only one statement. So we could have written the previous example this way ( Bad):

if (x % 2 == 0)
    System.out.println("x is even");
else
    System.out.println("x is odd");

The switch statement can have a number of possible execution paths. A switch works with the byte, short, char, and int primitive data types. It also works with enums, String.

Here is an example (adapted from -- Java Tutorial):

public class SwitchDemo {
    public static void main(String[] args) {

        int month = 8;
        String monthString;
        switch (month) {
        case 1:  monthString = "January";
            break;
        case 2:  monthString = "February";
            break;
        case 3:  monthString = "March";
            break;
        case 4:  monthString = "April";
            break;
        case 5:  monthString = "May";
            break;
        case 6:  monthString = "June";
            break;
        case 7:  monthString = "July";
            break;
        case 8:  monthString = "August";
            break;
        case 9:  monthString = "September";
            break;
        case 10: monthString = "October";
            break;
        case 11: monthString = "November";
            break;
        case 12: monthString = "December";
            break;
        default: monthString = "Invalid month";
            break;
        }
        System.out.println(monthString);
    }
}

August

Here’s an example of adding more methods to a class:

public class PrintTwice {

    public static void printTwice(String s) {
        System.out.println(s);
        System.out.println(s);
    }

    public static void main(String[] args) {
        String sentence = “Polly likes crackers”
        printTwice(sentence);

    }
}

Polly likes crackers
Polly likes crackers

By convention, method names should be named in the camelCase format.

Similar to the main method, the printTwice method is public (i.e., it can be invoked from other classes) static and void.

Parameters

A method can specify parameters. The printTwice method above specifies a parameter of String type. The main method passes the argument "Polly likes crackers" to that parameter.

The value provided as an argument must have the same type as the parameter. Sometimes Java can convert an argument from one type to another automatically. For example, if the method requires a double, you can invoke it with an int argument 5 and Java will automatically convert the argument to the equivalent value of type double 5.0.

Because a variable declared inside a method only exists inside that method, such variables are called local variables. That applies to parameters of a method too. For example, In the code above, s cannot be used inside main because it is a parameter of the printTwice method and can only be used inside that method. If you try to use s inside main, you’ll get a compiler error. Similarly, inside printTwice there is no such thing as sentence. That variable belongs to main.

The return statement allows you to terminate a method before you reach the end of it:

public static void printLogarithm(double x) {
    if (x <= 0.0) {
        System.out.println("Error: x must be positive.");
        return;
    }
    double result = Math.log(x);
    System.out.println("The log of x is " + result);
}

It can be used to return a value from a method too:

public class AreaCalculator{

    public static double calculateArea(double radius) {
        double result = 3.14 * radius * radius;
        return result;
    }

    public static void main(String[] args) {
        double area = calculateArea(12.5);
        System.out.println(area);
    }
}

Java methods can be overloaded. If two methods do the same thing, it is natural to give them the same name. Having more than one method with the same name is called overloading, and it is legal in Java as long as each version has a different method signature (the signature of the method is the method name and ordered list of parameter types) . For example, the following overloading of the method calculateArea is allowed because the method signatures are different (i.e., calculateArea(double) vs calculateArea(double, double)).

public static double calculateArea(double radius) {
    //...
}

public static double calculateArea(double height, double width) {
    //...
}

Methods can be recursive. Here is an example in which the nLines method calls itself recursively:

public static void nLines(int n) {
    if (n > 0) {
        System.out.println();
        nLines(n - 1);
    }
}

Here is an example while loop:

public static void countdown(int n) {
    while (n > 0) {
        System.out.println(n);
        n = n - 1;
    }
    System.out.println("Blastoff!");
}

for loops have the form:

for (initializer; condition; update) {
    statement(s);
}

Here is an example:

public static void printTable(int rows) {
    for (int i = 1; i <= rows; i = i + 1) {
        printRow(i, rows);
    }
}

The while and for statements are pretest loops; that is, they test the condition first and at the beginning of each pass through the loop. Java also provides a posttest loop: the do-while statement. This type of loop is useful when you need to run the body of the loop at least once.

Here is an example (from -- Java Tutorial):

class DoWhileDemo {
    public static void main(String[] args){
        int count = 1;
        do {
            System.out.println("Count is: " + count);
            count++;
        } while (count < 11);
    }
}

A break statement exits the current loop.

Here is an example (from -- Java Tutorial):

class Main {
    public static void main(String[] args) {
        int[] numbers = new int[] { 1, 2, 3, 0, 4, 5, 0 };
        for (int i = 0; i < numbers.length; i++) {
            if (numbers[i] == 0) {
                break;
            }
            System.out.print(numbers[i]);
        }
    }
}

123

[Try the above code on Repl.it]

A continue statement skips the remainder of the current iteration and moves to the next iteration of the loop.

Here is an example (from -- Java Tutorial):

public static void main(String[] args) {
    int[] numbers = new int[] { 1, 2, 3, 0, 4, 5, 0 };
    for (int i = 0; i < numbers.length; i++) {
        if (numbers[i] == 0) {
            continue;
        }
        System.out.print(numbers[i]);
    }
}

12345

[Try the above code on Repl.it]

Since traversing arrays is so common, Java provides an alternative for-loop syntax that makes the code more compact. For example, consider a for loop that displays the elements of an array on separate lines:

for (int i = 0; i < values.length; i++) {
    int value = values[i];
    System.out.println(value);
}

We could rewrite the loop like this:

for (int value : values) {
    System.out.println(value);
}

This statement is called an enhanced for loop. You can read it as, “for each value in values”. Notice how the single line for (int value : values) replaces the first two lines of the standard for loop.

Java comes with many inbuilt classes which are organized into packages. Here are some examples:

Before using a class in your code, you need to import the class. import statements appear at the top of the code.

import java.awt.Point;

public class Main{
    public static void main(String[] args) {
        Point spot = new Point(3, 4);
        int x = spot.x;
        System.out.println(x);
   }
}

You might wonder why we can use the System class without importing it. System belongs to the java.lang package, which is imported automatically.

To create a new object, you have to use the new operator

Point spot = new Point(3, 4);

null is a special value that means "no object". You can assign null to a variable to indicate that the variable is 'empty' at the moment. However, if you try to use a null value, either by accessing an attribute or invoking a method, Java throws a NullPointerException.

Point spot = null;
int x = spot.x;          // NullPointerException
spot.translate(50, 50);  // NullPointerException

On the other hand, it is legal to return null from a method or to pass a null reference as an argument to a method.

public static Point createCopy(Point p) {
    if (p == null) {
        return null; // return null if p is null
    }

    // create a new object with same x,y values
    return new Point(p.x, p.y);
}

Point result = createCopy(null);
System.out.println(result);

null

The syntax for is similar to that of other methods, except:

• The name of the constructor is the same as the name of the class.
• The keyword static is omitted.
• Does not return anything. A constructor returns the created object by default.

When you invoke new, Java creates the object and calls your constructor to initialize the instance variables. When the constructor is done, it returns a reference to the new object.

public Time() {
    hour = 0;
    minute = 0;
    second = 0;
}

Like other methods, constructors can be .

public Time(int h, int m, int s) {
    hour = h;
    minute = m;
    second = s;
}

The this keyword is a reference variable in Java that refers to the . You can use this the same way you use the name of any other object. For example, you can read and write the instance variables of this, and you can pass this as an argument to other methods. But you do not declare this, and you can’t make an assignment to it.

public Time(int hour, int minute, int second) {
    this.hour = hour;
    this.minute = minute;
    this.second = second;
}

this can be used to refer to a constructor of a class within the same class too.

public Time() {
    this(0, 0, 0); // call the overloaded constructor
}

public Time(int hour, int minute, int second) {
    // ...
}

Instance methods

You can add methods to a class which can then be used from the objects of that class. These instance methods do not have the static keyword in the method signature. Instance methods can access attributes of the class.

public int secondsSinceMidnight() {
    return hour*60*60 + minute*60 + second;
}

Time t = new Time(0, 2, 5);
System.out.println(t.secondsSinceMidnight() + " seconds since midnight!");

When a number of objects are created from the same class blueprint, they each have their own distinct copies of instance variables. In the case of a Bicycle class, the instance variables are gear, and speed. Each Bicycle object has its own values for these variables, stored in different memory locations.

Sometimes, you want to have variables that are common to all objects. This is accomplished with the static modifier. Fields that have the static modifier in their declaration are called static fields or class variables. They are associated with the class, rather than with any object. Every instance of the class shares a class variable, which is in one fixed location in memory. Any object can change the value of a class variable, but class variables can also be manipulated without creating an instance of the class.

public class Bicycle {

    private int gear;
    private int speed;

    // an instance variable for the object ID
    private int id;

    // a class variable for the number of Bicycle
    //   objects instantiated
    private static int numberOfBicycles = 0;
        ...
}

Class variables are referenced by the class name itself, as in Bicycle.numberOfBicycles This makes it clear that they are class variables.

The Java programming language supports static methods as well as static variables. Static methods, which have the static modifier in their declarations, should be invoked with the class name, without the need for creating an instance of the class, as in ClassName.methodName(args)

The static modifier, in combination with the final modifier, is also used to define constants. The final modifier indicates that the value of this field cannot change. For example, the following variable declaration defines a constant named PI, whose value is an approximation of pi (the ratio of the circumference of a circle to its diameter): static final double PI = 3.141592653589793;

public class Bicycle {

    private int gear;
    private int speed;

    private int id;

    private static int numberOfBicycles = 0;


    public Bicycle(int startSpeed, int startGear) {
        gear = startGear;
        speed = startSpeed;

        numberOfBicycles++;
        id = numberOfBicycles;
    }

    public int getID() {
        return id;
    }

    public static int getNumberOfBicycles() {
        return numberOfBicycles;
    }

    public int getGear(){
        return gear;
    }

    public void setGear(int newValue) {
        gear = newValue;
    }

    public int getSpeed() {
        return speed;
    }

    // ...

}

Strings provide a method named charAt, which extracts a character. It returns a char, a primitive type that stores an individual character (as opposed to strings of them).

String fruit = "banana";
char letter = fruit.charAt(0);

The argument 0 means that we want the letter at position 0. Like array indexes, string indexes start at 0, so the character assigned to letter is 'b'.

You can convert a string to an array of characters using the toCharArray method.

char[] fruitChars = fruit.toCharArray()

Strings provide methods, toUpperCase and toLowerCase, that convert from uppercase to lowercase and back.

String name = "Alan Turing";
String upperName = name.toUpperCase();
System.out.println(name);
System.out.println(upperName);

Alan Turing
ALAN TURING

Note that a string method cannot change the string object on which the method is invoked, because strings are . For example, when you invoke toUpperCase on a string "abc", you get a new string object "ABC" as the return value rather than the string "abc" being changed to "ABC". As a result, for such string methods that seemingly modify the string but actually return a new string instead e.g., toLowerCase, invoking the method has no effect if you don’t assign the return value to a variable.

String s = "Ada";
s.toUpperCase(); // no effect
s = s.toUpperCase(); // the correct way

Another useful method is replace, which finds and replaces instances of one string within another.

String text = "Computer Science is fun!";
text = text.replace("Computer Science", "CS");
System.out.println(text);

CS is fun!

The substring method returns a new string that copies letters from an existing string, starting at the given index.

• "banana".substring(0) "banana"
• "banana".substring(2) "nana"
• "banana".substring(6) ""

If it’s invoked with two arguments, they are treated as a start and end index:

• "banana".substring(0, 3) "ban"
• "banana".substring(2, 5) "nan"
• "banana".substring(6, 6) ""

The indexOf method searches for a single character (or a substring) in a string and returns the index of the first occurrence. The method returns -1 if there are no occurrences.

• "banana".indexOf('a') 1
• "banana".indexOf('a', 2) 3 searches for 'a', starting from position 2
• "banana".indexOf('x') -1
• "banana".indexOf("nan") 2 searches for the substring "nan"

To compare two strings, it is tempting to use the == and != operators.

String name1 = "Alan Turing";
String name2 = "Alan Turing";
System.out.println(name1 == name2);

This code compiles and runs, and most of the time it shows true. But it is not correct. The problem is, , the == operator checks whether the two variables refer to the same object (by comparing the references). If you give it two different string objects that contain the same letters, it is supposed to yield false because they are two distinct objects even if they contain the same text. However, because Java strings are immutable, in some cases (but not always) Java reuses existing string objects instead of creating multiple objects, which can cause the above code to yield true. Therefore, it is not safe to use == to compare strings if your intention is to check if they contain the same text.

The right way to compare strings is with the equals method.

if (name1.equals(name2)) {
    System.out.println("The names are the same.");
}

If the strings differ, you can use compareTo to see which comes first in alphabetical order. The return value from compareTo is the difference between the first characters in the strings that differ. If the strings are equal, their difference is zero. If the first string (the one on which the method is invoked) comes first in the alphabet, the difference is negative. Otherwise, the difference is positive.

String name1 = "Alan";
String name2 = "Ada";
int diff = name1.compareTo(name2);
if (diff == 0) {
    System.out.println("The names are the same.");
} else if (diff < 0) {
    System.out.println("name1 comes before name2.");
} else if (diff > 0) {
    System.out.println("name2 comes before name1.");
}

Both equals and compareTo are case-sensitive. The uppercase letters come before the lowercase letters, so "Ada" comes before "ada". To check if two strings are similar irrespective of the differences in case, you can use the equalsIgnoreCase method.

String s1 = "Apple";
String s2 = "apple";
System.out.println(s1.equals(s2)); //false
System.out.println(s1.equalsIgnoreCase(s2)); //true

Some more comparison-related String methods:

• contains: checks if one string is a sub-string of the other e.g., Snapple and app
• startsWith: checks if one string has the other as a substring at the beginning e.g., Apple and App
• endsWith: checks if one string has the other as a substring at the end e.g., Crab and ab

You can embed a special character e.g., line break, tab, backspace, etc. in a string using an escape sequence.

System.out.println("First line\nSecond \"line\"");

First line
Second "line"

As the behavior of the \n , the recommended way to print a line break is using the System.lineSeparator() as it works the same in all platforms.

System.out.println("First" + System.lineSeparator() + "Second");

First
Second

Sometimes programs need to create strings that are formatted in a certain way. String.format takes a format specifier followed by a sequence of values and returns a new string formatted as specified.

public static String timeString(int hour, int minute) {
    String ampm;
    if (hour < 12) {
        ampm = "AM";
        if (hour == 0) {
            hour = 12;  // midnight
        }
    } else {
        ampm = "PM";
        hour = hour - 12;
    }

    // returns "07:05 PM"
    return String.format("%02d:%02d %s", hour, minute, ampm);
}

A class that is derived from another class is called a subclass (also a derived class, extended class, or child class). The class from which the subclass is derived is called a superclass (also a base class or a parent class).

A subclass inherits all the members (fields, methods, and nested classes) from its superclass. Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass.

Every class has one and only one direct superclass (single inheritance), except the Object class, which has no superclass, . In the absence of any other explicit superclass, every class is implicitly a subclass of Object. Classes can be derived from classes that are derived from classes that are derived from classes, and so on, and ultimately derived from the topmost class, Object. Such a class is said to be descended from all the classes in the inheritance chain stretching back to Object. Java does not support multiple inheritance among classes.

The java.lang.Object class defines and implements behavior common to all classes—including the ones that you write. In the Java platform, many classes derive directly from Object, other classes derive from some of those classes, and so on, forming a single hierarchy of classes.

The keyword extends indicates one class inheriting from another.

public class Bicycle {

    public int gear;
    public int speed;

    public Bicycle(int startSpeed, int startGear) {
        gear = startGear;
        speed = startSpeed;
    }

    public void setGear(int newValue) {
        gear = newValue;
    }

    public void applyBrake(int decrement) {
        speed -= decrement;
    }

    public void speedUp(int increment) {
        speed += increment;
    }

}

public class MountainBike extends Bicycle {

    // the MountainBike subclass adds one field
    public int seatHeight;

    // the MountainBike subclass has one constructor
    public MountainBike(int startHeight, int startSpeed, int startGear) {
        super(startSpeed, startGear);
        seatHeight = startHeight;
    }

    // the MountainBike subclass adds one method
    public void setHeight(int newValue) {
        seatHeight = newValue;
    }
}

A subclass inherits all the fields and methods of the superclass. In the example above, MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it.

Accessing superclass members

If your method overrides one of its superclass's methods, you can invoke the overridden method through the use of the keyword super. You can also use super to refer to a (although hiding fields is discouraged).

public class Superclass {

    public void printMethod() {
        System.out.println("Printed in Superclass.");
    }
}

public class Subclass extends Superclass {

    // overrides printMethod in Superclass
    public void printMethod() {
        super.printMethod();
        System.out.println("Printed in Subclass");
    }
    public static void main(String[] args) {
        Subclass s = new Subclass();
        s.printMethod();
    }
}

Printed in Superclass.
Printed in Subclass

Subclass constructors

A subclass constructor can invoke the superclass constructor. Invocation of a superclass constructor must be the first line in the subclass constructor. The syntax for calling a superclass constructor is super() (which invokes the no-argument constructor of the superclass) or super(parameters) (to invoke the superclass constructor with a matching parameter list).

public MountainBike(
        int startHeight, int startSpeed, int startGear) {

    super(startSpeed, startGear);
    seatHeight = startHeight;
}

Note: If a constructor does not explicitly invoke a superclass constructor, the Java compiler automatically inserts a call to the no-argument constructor of the superclass. If the superclass does not have a no-argument constructor, you will get a compile-time error. Object does have such a constructor, so if Object is the only superclass, there is no problem.

Access modifiers (simplified)

Access level modifiers determine whether other classes can use a particular field or invoke a particular method. Given below is a simplified version of Java access modifiers, assuming you have not yet started placing your classes in different packages i.e., all classes are placed in the root level. A full explanation of access modifiers is given in a later topic.

There are two levels of access control:

1. At the class level:

   • public: the class is visible to all other classes
   • no modifier: same as public
     
     

2. At the member level:

   • public : the member is visible to all other classes
   • protected: same as public
   • no modifier: same as public
   • private: the member is not visible to other classes (but can be accessed in its own class)

Every class inherits a toString method from the Object class that is used by Java to get a string representation of the object e.g., for printing. By default, it simply returns the type of the object and its address (in hexadecimal).

class Time {
    int hours;
    int minutes;
    int seconds;

    Time(int hours, int minutes, int seconds) {
        this.hours = hours;
        this.minutes = minutes;
        this.seconds = seconds;
    }
}

 Time t = new Time(5, 20, 13);
 System.out.println(t);

You can override the toString method in your classes to provide a more meaningful string representation of the objects of that class.

class Time{

     //...

     @Override
     public String toString() {
         return String.format("%02d:%02d:%02d\n",
                 this.hours, this.minutes, this.seconds);
     }
}

 Time t = new Time(5, 20, 13);
 System.out.println(t);

@Override is an optional annotation you can use to indicate that the method is overriding a method from the parent class.

There are two ways to check whether values are equal: the == operator and the equals method. With objects you can use either one, but they are not the same.

• The == operator checks whether objects are identical; that is, whether they are the same object.
• The equals method checks whether they are equivalent; that is, whether they have the same value.

The definition of identity is always the same, so the == operator always does the same thing.

Time time1 = new Time(9, 30, 0);
Time time2 = time1;
Time time3 = new Time(9, 30, 0);

By default, the equals method inherited from the Object class does the same thing as ==. As the definition of equivalence is different for different classes, you can override the equals method to define your own criteria for equivalence of objects of your class.

public class Time {
    int hours;
    int minutes;
    int seconds;

    // ...

    @Override
    public boolean equals(Object o) {
        Time other = (Time) o;
        return this.hours == other.hours
                && this.minutes == other.minutes
                && this.seconds == other.seconds;
    }
}

Time t1 = new Time(5, 20, 13);
Time t2 = new Time(5, 20, 13);
System.out.println(t1 == t2);
System.out.println(t1.equals(t2));

false
true