How do I invoke superclass constructor?

By I Wayan Saryada in Basic, Core API 0 Comment

This example shows you how to use the super keyword to call a superclass constructor. The Female class constructor calls its superclass constructor and initializes its own initialization parameters. The call to the superclass constructor must be done in the first line of the constructor in the subclass.

package org.kodejava.example.fundamental;

public class Human {
    private String gender;
    private int age;

    public Human(String gender) {
        this.gender = gender;
    }
}

To call a superclass constructor we call super(). In the case below we call the superclass constructor with one string variable as a parameter.

package org.kodejava.example.fundamental;

public class Female extends Human {
    private String hairStyle;

    public Female(String hairStyle, String gender) {
        super(gender);
        this.hairStyle = hairStyle;
    }
}

How do I create and implement abstract class?

By I Wayan Saryada in Basic, Core API 0 Comment

Abstract class is a class that have one or more methods declared, but not defined. Abstract class cannot have instances. This class uses in inheritance to take advantage of polymorphism. To declare that a class is abstract, use the abstract keyword in front of the class keyword in the class definition.

Methods in abstract class that have no definition are called abstract methods. The declaration for an abstract method ends with a semicolon and you specify the method with the abstract keyword to identify it as such. The implementation is left to the sub classes.

package org.kodejava.example.fundamental;

public abstract class Animal {
    private String species;

    public Animal(String species) {
        this.species = species;
    }

    public abstract void makeASound();

    public String getSpecies() {
        return species;
    }

    public static void main(String[] args) {
        Animal pig = new Pig("Warthog");
        pig.makeASound();
    }
}

The Pig class extends the Animal class. Because the Animal class contains an abstract method makeASound() the Pig class must implements this method or else the Pig will also become an abstract class.

package org.kodejava.example.fundamental;

public class Pig extends Animal {

    public Pig(String species) {
        super(species);
    }

    @Override
    public void makeASound() {
        System.out.println("oink oink");
    }
}

How do I override a method in Java?

By I Wayan Saryada in Basic, Core API 0 Comment

If a subclass has a method that have the same signature with a method in its superclass, then it’s an overriding method. Override inherited methods allow subclasses to provide specialized implementation for those methods. The overriding method has the same name, number and type of arguments, and return value as the method it overrides.

The overriding method can have a different throws clause as long as it doesn’t specify any types not specified by the throws clause in the overridden method. Also, the access specifier for the overriding method can allow more but not less access than the overridden method.

For example, a protected method in the superclass can be made public but not private. A subclass cannot override methods that are declared final in the superclass. A subclass must override methods that are declared abstract in the superclass, or the subclass itself must be abstract.

package org.kodejava.example.fundamental;

public class Car {
    private String type;
    private String brand;
    private String model;
    private int numberOfSeat;

    public Car() {
    }

    public Car(String type, String brand, String model) {
        this.type = type;
        this.brand = brand;
        this.model = model;
    }

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getBrand() {
        return brand;
    }

    public void setBrand(String brand) {
        this.brand = brand;
    }

    public String getModel() {
        return model;
    }

    public void setModel(String model) {
        this.model = model;
    }

    public int getNumberOfSeat() {
        return numberOfSeat;
    }

    public void setNumberOfSeat(int numberOfSeat) {
        this.numberOfSeat = numberOfSeat;
    }

    public String getCarInfo() {
        return"Type: " + type
                + "; Brand: " + brand
                + "; Model: " + model;
    }
}

In the Truck class we override the getCarInfo() method to provide more information about the truck object. We can also add an @Override annotation to an overriding method.

package org.kodejava.example.fundamental;

public class Truck extends Car {
    private int loadCapacity;

    public Truck() {
    }

    public Truck(String type, String brand, String model) {
        super(type, brand, model);
    }

    public int getLoadCapacity() {
        return loadCapacity;
    }

    public void setLoadCapacity(int loadCapacity) {
        this.loadCapacity = loadCapacity;
    }

    @Override
    public String getCarInfo() {
        return "Type: " + getType()
                + "; Brand: " + getBrand()
                + "; Model: " + getModel()
                + "; Load capacity: " + getLoadCapacity();
    }
}

How do I declare and initialize variable?

By I Wayan Saryada in Basic, Core API 0 Comment

Variable is a field in which object store its state. It also an allocations for the placement of data in memory. When the statement of variable declaration is compiled, some bytes of memory will be allocated for the variable. The size is determined by the type of variable.

One variable definition is able to store data only of one particular type. Before it can use, the variable must be declared. The name and type of variable must be specified in variable declaration. If you want the variable to have an initial value, you must specify your own value in the declaration.

You can assign a value into variable by using an assignment statement. The assignment operator is =.

package org.kodejava.basic;

public class VariableExample {
    // declares a double variable number1 and total
    private double number1, total;

    // declares a double variable and initializes its value to 10000
    private double number2 = 1000;


    public static void main(String[] args) {
        VariableExample ve = new VariableExample();

        // assigns a value to variable number1
        ve.number1 = 500;

        // assigns the calculation result of number1 + number2 to 
        // total
        ve.total = ve.number1 + ve.number2;
        System.out.println(ve.total);
    }
}

How do I declare and initialize local variable?

By I Wayan Saryada in Basic, Core API 0 Comment

Local variables are variables that are not fields of a class. A function or method often store its temporary state in local variables. Local variables only visible to the methods in which they are declared.

Local variables must be declared and initialized before it used for the first time. Local variables will not get a default value if you do not initialize it and could cause a compile-time error.

package org.kodejava.basic;

public class LocalVariableExample {
    // it's okay if total variable does not initialize.
    // it will initialize with default value = 0.
    int total;

    public static int add() {
        // this will cause compile-time error if does not initialize
        int x = 1, y = 2;

        // z is assigned by the calculation result of x + y
        int z = x + y;
        return z;
    }

    public static void main(String[] args) {
        LocalVariableExample lve = new LocalVariableExample();
        // assigns total with the result of add() method execution
        lve.total = add();
        System.out.println("total= " + lve.total);
    }
}