How do I use the equality operator in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

Equality operator is used to compare two similar things (numbers, characters, boolean, primitives and object references). Equality operator will always result in boolean value (true or false).

For object reference, it will return true if only both reference variables refer to the same object.

package org.kodejava.basic;

public class EqualityDemo {
    public static void main(String[] args) {
        int value1 = 10, value2 = 10, number1 = 10;
        char a = 'a', b = 'b';
        double number2 = 10d;
        Cat kitty = new Cat("Kitty");
        Cat kitten = new Cat("Kitty");
        Cat sweetie = kitty;

        if (value1 == value2) {
            System.out.println("Equal");
        }

        if (a != b) {
            System.out.println("Not Equal");
        }

        // though it have different type, but it have same value
        if (number1 == number2) {
            System.out.println("Equal");
        }

        // it's not refer to the same object, so it will return
        // false
        if (kitty == kitten) {
            System.out.format("(kitty == kitten) = " + (kitty == kitten));
        } else {
            System.out.println("(kitty == kitten) = " + (kitty == kitten));
        }

        // it's refer to the same object, so it will return true
        if (kitty == sweetie) {
            System.out.println("(kitty == sweetie) = " + (kitty == sweetie));
        }

        if (true != false) {
            System.out.println("true != false");
        }

    }
}

class Cat {
    private String name;

    Cat(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

And here are the result of the program:

Equal
Not Equal
Equal
(kitty == kitten) = false
(kitty == sweetie) = true
true != false

How do I use the && operator in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

The && operator also known as conditional-AND operator or short circuit AND. This operator exhibit “short-circuiting” behavior, which means that the second operand is evaluated only if needed.

The && operator evaluate only boolean values. For an AND (&&) expression to be true, both operands must be true. If the first operand resolves false, then the && operator will not evaluate the second operand, because it already know the complete expression will return false.

package org.kodejava.basic;

public class ConditionalANDDemo {
    public static void main(String[] args) {
        // second operand (2<3) is not evaluated, because the first
        // operand return false the result of complete expression
        // can't be true
        boolean a = (5 < 3) && (2 < 3);

        // first operand return true, second operand is evaluated
        // to check the result of the second expression if second
        // operand resolves to true, the complete expression return
        // false, otherwise return false
        boolean b = (1 == 1) && (2 < 3);
        boolean c = (1 == 1) && (5 < 3);

        System.out.println("result a: " + a);
        System.out.println("result b: " + b);
        System.out.println("result c: " + c);
    }
}

The result of the code snippet:

result a: false
result b: true
result c: false

What is reference variable in Java?

By I Wayan Saryada in Basic, Core API 6 Comments

The only way you can access an object is through a reference variable. A reference variable is declared to be of a specific type and that type can never be changed. Reference variables can be declared as static variables, instance variables, method parameters, or local variables.

A reference variable that is declared as final can’t never be reassigned to refer to a different object. The data within the object can be modified, but the reference variable cannot be changed.

package org.kodejava.basic;

public class ReferenceDemo {
    public static void main(String[] args) {
        // Declaration of Reference variable
        Reference ref1, ref2;

        // ref3 is declared final, ref3 can't be reassigned
        // or refer to different object
        final Reference ref3;

        // assign ref1 with object Reference
        ref1 = new Reference("This is the first reference variable", 1);

        // access method getNumber() of object Reference through
        // variable ref1
        int number = ref1.getNumber();
        System.out.println("number= " + number);

        // assign ref2 with object Reference
        ref2 = new Reference("This is the second reference variable", 2);

        // passing ref2 as method parameter of printText() method
        ReferenceDemo.printText(ref2);

        // assign ref3 with object Reference
        ref3 = new Reference("This is the third reference variable", 3);

        // try to reassign ref3 will cause a compile-time error
        // ref3 = new Reference("Try to reassign", 3);

    }

    public static void printText(Reference reference) {
        String text = reference.getText();
        System.out.println(text);
    }
}

package org.kodejava.basic;

public class Reference {
    private int number;
    private String text;

    Reference(String text, int number) {
        this.text = text;
        this.number = number;
    }

    public String getText() {
        return text;
    }

    public int getNumber() {
        return number;
    }
}

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How do I use the instanceof keyword?

By I Wayan Saryada in Basic, Core API 0 Comment

To check whether an object is of a particular type (class or interface type) you can use instanceof operator. The instanceof operator is used only for object reference variable. x instanceof y can be read as x is-a y.

The instanceof returns true if the reference variable being tested is of the type being compared to. It will still return true if the object being compared is assignment compatible with the type on the right.

For interface type, an object is said to be of a particular interface type (meaning it will pass the instanceof test) if any of the object’s superclasses implement the interface.

package org.kodejava.basic;

interface Man {
}

public class InstanceofDemo {
    public static void main(String[] args) {
        Body body = new Body();
        Hand hand = new Hand();
        Nail nail = new Nail();
        Shoes shoe = new Shoes();

        if (body instanceof Man) {
            System.out.println("body is a Man");
        }

        if (hand instanceof Man) {
            System.out.println("hand is a Man too");
        }

        if (hand instanceof Body) {
            System.out.println("hand is a Body");
        }

        // it should be return false
        if (hand instanceof Nail) {
            System.out.println("hand is a Nail");
        } else {
            System.out.println("hand is not a Nail");
        }

        if (nail instanceof Man) {
            System.out.println("nail is a Man too");
        }

        if (nail instanceof Hand) {
            System.out.println("nail is a Hand");
        }
        if (nail instanceof Body) {
            System.out.println("nail is a Body too");
        }

        // it should return false, cause Shoes is not implements Man
        if (shoe instanceof Man) {
            System.out.println("shoe is a Man");
        } else {
            System.out.println("shoe is not a Man");
        }

        // compile error. cannot test against class in different
        // class hierarchies.
        //
        //if (shoe instanceof Body) {
        //}

    }

}

class Body implements Man {
}

// indirect implements Man
class Hand extends Body {
}

// indirect implements Man
class Nail extends Hand {
}

class Shoes {
}

The result of the code snippet above:

body is a Man
hand is a Man too
hand is a Body
hand is not a Nail
nail is a Man too
nail is a Hand
nail is a Body too
shoe is not a Man

How do I define a constant variable?

By I Wayan Saryada in Basic, Core API 0 Comment

To define a constant in Java, use final modifier which combined with static modifier. The final modifier indicates that the value of this field cannot change.

If you change the value of the constant, you need to recompile the class to get the current value. Other feature in Java that provide similar functionality is enumeration (a list of named constants). You can simply create an enumeration by using the enum keyword.

package org.kodejava.example.fundamental;

public class ConstantDemo {
    public static void main(String[] args) {
        int sunday = DayConstant.SUNDAY;
        System.out.println("Sunday = " + sunday);

        String dozen = MeasureConstant.DOZEN;
        System.out.println("Dozen  = " + dozen);
    }
}

class DayConstant {
    public final static int SUNDAY = 0;
    public final static int MONDAY = 1;
    public final static int TUESDAY = 2;
    public final static int WEDNESDAY = 3;
    public final static int THURSDAY = 4;
    public final static int FRIDAY = 5;
    public final static int SATURDAY = 6;
}

class MeasureConstant {
    final static String UNIT = "unit";
    final static String DOZEN = "dozen";
}