How do I override a method in Java?

By Wayan 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 overload methods in Java?

By Wayan in Basic, Core API 0 Comment

Method overloading allows a method to use the same name or identifier as the method name as long as the argument list is different. Java can differentiate each method by their method signatures. For example to print some value you can create a print method that accept different kind of objects or values as its parameters.

Overloaded method is differentiated by the number and the type of argument they accept. The print(String string) and print(int number) are distinct and unique due to their argument type.

The compiler does not count a return type as a method differentiator. So it is not legal to create a method with the same name, the same number, the same type of argument but with a different return type.

package org.kodejava.basic;

public class OverloadedExample {
    public void print(Object object) {
        System.out.println("object = " + object);
    }

    public void print(String string) {
        System.out.println("string = " + string);
    }

    public void print(int number) {
        System.out.println("number = " + number);
    }

    public void print(float number) {
        System.out.println("number = " + number);
    }

    public void print(double number) {
        System.out.println("number = " + number);
    }
}

How do I annotate a class or method?

By Wayan in Core API, Lang Package 0 Comment

This example show you how to use the HelloAnnotation annotation on the previous example code, How do I create a simple annotation?. We add the HelloAnnotation annotation to our class and its methods.

package org.kodejava.lang.annotation;

@HelloAnnotation(value = "Good Morning", greetTo = "Universe")
public class HelloAnnotationExample {
    public static void main(String[] args) {
        HelloAnnotationExample hello = new HelloAnnotationExample();
        hello.sayHi();
        hello.sayHello();
    }

    @HelloAnnotation(value = "Hi there", greetTo = "Alice")
    private void sayHi() {
    }

    @HelloAnnotation(value = "Hello there", greetTo = "Bob")
    private void sayHello() {
    }
}

How do I invoke a method using Method class?

By Wayan in Reflection 0 Comment

This example demonstrate using reflection to invoke methods of a class object. Using reflection we can call method of an object using the given string name of the method. When using this method we need to catch for the NoSuchMethodException, IllegalAccessException and InvocationTargetException.

package org.kodejava.lang.reflect;

import java.lang.reflect.Method;
import java.lang.reflect.InvocationTargetException;

public class InvokingMethod {
    public static void main(String[] args) {
        InvokingMethod object = new InvokingMethod();
        Class<? extends InvokingMethod> clazz = object.getClass();

        try {
            // Invoking the add(int, int) method
            Method method = clazz.getMethod("add", int.class, int.class);
            Object result = method.invoke(object, 10, 10);
            System.out.println("Result = " + result);

            // Invoking the multiply(int, int) method
            method = clazz.getMethod("multiply", int.class, int.class);
            result = method.invoke(object, 10, 10);
            System.out.println("Result = " + result);

        } catch (NoSuchMethodException | IllegalAccessException | InvocationTargetException e) {
            e.printStackTrace();
        }
    }

    public int add(int numberA, int numberB) {
        return numberA + numberB;
    }

    public int multiply(int numberA, int numberB) {
        return numberA * numberB;
    }
}

How do I get the methods of a class object?

By Wayan in Reflection 0 Comment

In this example we show you how to get the methods available in a class object. We show three ways to get the methods, they are:

  • Class.getDeclaredMethods()
  • Class.getMethods()
  • Class.getMethod(String, Class<?>...)
package org.kodejava.lang.reflect;

import java.lang.reflect.Method;

public class GetMethods {
    public static void main(String[] args) {
        GetMethods object = new GetMethods();
        Class<? extends GetMethods> clazz = object.getClass();

        // Get all declared methods including public, protected, private and
        // package (default) access but excluding the inherited methods.
        Method[] methods = clazz.getDeclaredMethods();
        for (Method method : methods) {
            System.out.println("Method name        = " + method.getName());
            System.out.println("Method return type = " + method.getReturnType().getName());

            Class<?>[] paramTypes = method.getParameterTypes();
            for (Class<?> c : paramTypes) {
                System.out.println("Param type         = " + c.getName());
            }

            System.out.println("----------------------------------------");
        }

        System.out.println(">>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>");

        // Get all methods including the inherited method. Using the getMethods()
        // we can only access public methods.
        methods = clazz.getMethods();
        for (Method method : methods) {
            System.out.println("Method name        = " + method.getName());
            System.out.println("Method return type = " + method.getReturnType().getName());

            Class<?>[] paramTypes = method.getParameterTypes();
            for (Class<?> c : paramTypes) {
                System.out.println("Param type         = " + c.getName());
            }

            System.out.println("----------------------------------------");
        }

        try {
            // We can also get method by their name and parameter types, here we
            // are trying to get the add(int, int) method.
            Method method = clazz.getMethod("add", int.class, int.class);
            System.out.println("Method name: " + method.getName());
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        }
    }

    public int add(int numberA, int numberB) {
        return numberA + numberB;
    }

    protected int multiply(int numberA, int numberB) {
        return numberA * numberB;
    }

    private double div(int numberA, int numberB) {
        return numberA / numberB;
    }
}