How do I use the instanceof keyword?

By Wayan 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 use the relational operator in Java?

By Wayan in Basic, Core API 0 Comment

Relational operators used to compare any combination of integers, floating-point numbers, or characters. The result of relational operators is always in a boolean value, true or false. It is mostly used in an if statement test.

There are four relational operators in Java:

  • > greater than
  • >= greater than or equal to
  • < less than
  • <= less than or equal to
package org.kodejava.basic;

public class RelationalDemo {
    public static void main(String[] args) {
        int value1 = 10, value2 = 25;
        int age = 15;
        double salary = 1000d;
        char char1 = 'd', char2 = 'f';

        if (value1 > value2) {
            System.out.format("%d is greater than %d %n", value1, value2);
        } else {
            System.out.format("%d is greater than %d %n", value2, value1);
        }

        if (age >= 12) {
            System.out.format("Hey, I am not a kid anymore %n");
        }

        if (char1 < char2) {
            System.out.format("%c is less than %c %n", char1, char2);
        } else {
            System.out.format("%c is less than %c %n", char2, char1);
        }

        if (salary <= 3000d) {
            System.out.println("Entry-level Staff");
        }
    }
}

An here are the result of the program:

25 is greater than 10 
Hey, I am not a kid anymore 
d is less than f 
Entry-level Staff

How do I use the equality operator in Java?

By Wayan 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 Wayan 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

How do I define a constant variable?

By Wayan 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";
}

How do I calculate the length of hypotenuse?

By Wayan in Core API, Math 0 Comment

Hypot is a mathematical function defined to calculate the length of the hypotenuse of a right-angle triangle. It was designed to avoid errors arising due to limited-precision calculations performed on computers.

From: Wikipedia

The Math.hypot(double x, double y) return the sqrt(x2+ y2) without intermediate overflow or underflow. The result will be same with this calculation Math.sqrt(Math.pow(x, 2) + Math.pow(y, 2)).

package org.kodejava.math;

public class HypotExample {
    public static void main(String[] args) {
        double number1 = 3.0d;
        double number2 = 5.0d;

        // calculate square root of total value of
        // number1 ^ 2 + number2 ^ 2
        double sqr = Math.hypot(number1, number2);

        System.out.println("Total value = " +
                (Math.pow(number1, 2) + Math.pow(number2, 2)));
        System.out.println("Square root = " + sqr);
    }
}

How do I calculate exponential function?

By Wayan in Core API, Math 1 Comment

In mathematics, the exponential function is the function ex, where e is the number (approximately 2.718281828) such that the function ex equals its own derivative. The function f(x) = ex at the point x = 0 is equal to 1.

package org.kodejava.math;

public class ExponentExample {

    public static void main(String[] args) {
        double x = 0.0d;

        // calculates e raised to the power of x (e^x)
        double fx = Math.exp(x);

        // calculates e raised to the power of x minus 1 (e^x - 1)
        double fxm1 = Math.expm1(x);

        System.out.println("fx   = " + fx);
        System.out.println("fxm1 = " + fxm1);
    }
}

How do I get the exponent of exponential function?

By Wayan in Core API, Math 0 Comment

The exponential function is f(x) = ex. The Math.getExponent() method is used to get the x value of the given parameter, in which the parameter is a result of exponential function calculation.

package org.kodejava.math;

public class GetExponent {
    public static void main(String[] args) {
        double fx = 1.0d;
        float fx1 = 1.0f;

        int x = Math.getExponent(fx);
        System.out.println("Exponent  = " + x);

        // argument in float
        int xf = Math.getExponent(fx1);
        System.out.println("Exponent1 = " + xf);
    }
}

How do I use the “return” keyword in Java?

By Wayan in Basic, Core API 5 Comments

The return keyword is used to return from a method when its execution is complete. When a return statement is reached in a method, the program returns to the code that invoked it.

A method can return a value or reference type or does not return a value. If a method does not return a value, the method must be declared void and it doesn’t need to contain a return statement.

If a method declare to return a value, then it must use the return statement within the body of method. The data type of the return value must match the method’s declared return type.

package org.kodejava.basic;

public class ReturnDemo {

    public static void main(String[] args) {
        int z = ReturnDemo.calculate(2, 3);
        System.out.println("z = " + z);

        Dog dog = new Dog("Spaniel", "Doggie");
        System.out.println(dog.getDog());
    }

    public static int calculate(int x, int y) {
        // return an int type value
        return x + y;
    }

    public void print() {
        System.out.println("void method");

        // it does not need to contain a return statement, but it
        // may do so
        return;
    }

    public String getString() {
        return "return String type value";

        // try to execute a statement after return a value will
        // cause a compile-time error.
        //
        // String error = "error";
    }
}

class Dog {
    private String breed;
    private String name;

    Dog(String breed, String name) {
        this.breed = breed;
        this.name = name;
    }

    public Dog getDog() {
        // return Dog type
        return this;
    }

    public String toString() {
        return "breed: " + breed.concat("name: " + name);
    }
}

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How do I use the “this” keyword in Java?

By Wayan in Basic, Core API 0 Comment

Every instance method has a variable with the name this that refers to the current object for which the method is being called. You can refer to any member of the current object from within an instance method or a constructor by using this keyword.

Each time an instance method is called, the this variable is set to reference the particular class object to which it is being applied. The code in the method will then relate to the specific members of the object referred to by this keyword.

package org.kodejava.basic;

public class RemoteControl {
    private String channelName;
    private int channelNum;
    private int minVolume;
    private int maxVolume;

    RemoteControl() {
    }

    RemoteControl(String channelName, int channelNum) {
        // use "this" keyword to call another constructor in the 
        // same class
        this(channelName, channelNum, 0, 0);
    }

    RemoteControl(String channelName, int channelNum, int minVol, int maxVol) {
        this.channelName = channelName;
        this.channelNum = channelNum;
        this.minVolume = minVol;
        this.maxVolume = maxVol;
    }

    public static void main(String[] args) {
        RemoteControl remote = new RemoteControl("ATV", 10);

        // when the following line is executed, the variable in
        // changeVolume() is referring to remote object.
        remote.changeVolume(0, 25);
    }

    public void changeVolume(int x, int y) {
        this.minVolume = x;
        this.maxVolume = y;
    }
}