java tutorial - Абстрактный класс и Interface в Java - учебник java - java programming - учиться java - java basics - java for beginners

// This is an abstract class.
// It must be declared as abstract because it has an abstract method.
public abstract class ClassA  {
 
    // This is an abstract method
    // It has no body.
    // Its access modifier is public
    public abstract void doSomething();
 
    // The access modifier of this method is protected.
    protected abstract String doNothing();
    // This method does not declare access modifier
    // It has the default access modifier.
    abstract void todo() ;
}
 
// This is an abstract class.
// It is declared as abstract, 
// though it does not have any abstract methods.
public abstract class ClassB   {
 
}

package org.wikitechy.tutorial.abs;
 
// Class implements all abstract methods of the parent class.
public class JavaCoding extends AbstractJob {
 
    public JavaCoding() {
    }
 
    // Implements abstract method of parent class.
    @Override
    public void doJob() {
        System.out.println("Coding Java...");
    }
 
    // Implements abstract method of parent.
    // Method with body.
    // Return name of Job.
    @Override
    public String getJobName() {
        return "Coding Java";
    }
 
}

package org.wikitechy.tutorial.abs;
 
// AbstractJob class has two abstract methods.
// This class implements only one abstract method of parent class. 
// Therefore it must be declared as abstract.
public abstract class ManualJob extends AbstractJob {
 
    public ManualJob() {
 
    }
 
    // Implements abstract method of parent class.
    @Override
    public String getJobName() {
        return "Manual Job";
    }
 
}

package org.wikitechy.tutorial.abs;
 
// This class inherited from ManualJob
// BuildHouse is not declared as abstract.
// so it must implements all remaining abstract methods.
public class BuildHouse extends ManualJob {
 
    public BuildHouse() {
 
    }
 
    // Implements abstract methods of the parent class.
    @Override
    public void doJob() {
        System.out.println("Build a House");
    }
 
}

package org.wikitechy.tutorial.abs;
 
public class JobDemo {
 
    public static void main(String[] args) {
 
        // Create AbstractJob object
        // from Constructor of JavaCoding.
        AbstractJob job1 = new JavaCoding();
 
        // Call doJob() method.
        job1.doJob();
 
        // getJobName is the abstract method in AbstractJob class.
        // But it was implemented in a certain subclass (JavaCoding).
        // So can call it.
        String jobName = job1.getJobName();
 
        System.out.println("Job Name 1= " + jobName);
 
        // Create AbstractJob object
        // from constructor of BuildHouse.
        AbstractJob job2 = new BuildHouse();
 
        job2.doJob();
 
        String jobName2 = job2.getJobName();
 
        System.out.println("Job Name 2= " + jobName2);
 
    }
}

// B is subclass of class A, or in other words B is extended from A.
// Java only allows a class to extends from only one another class.
public class B extends A  {
   // ....
}
 
// In the absence of specifying class B extends from a particular class.
// By default, understand that B extends from the Object class.
public class B  {
 
}
 
// This declaration, and the above is equivalent.
public class B extends Object  {
 
}

// A class can extends from only one parent class.
// But can implements (extends) from multiple Interfaces.
public class Cat extends Animal implements CanEat, CanDrink {
 
  // ....
}

package org.wikitechy.tutorial.itf;
 
// This is an interface not specified access modifier.
// Its access modifier is default.
// Only classes in the same package can implements this interface.
interface NoAccessModifierInterface {
 
}

package org.wikitechy.tutorial.itf;
 
// This interface defines a standard, 
// about things capable of moving.
public interface CanMove {
 
    // The methods in Interface are always abstract and public.
    public abstract void run();
 
    // Even if you do not specify 'public abstract', java always understands.
    void back();
 
    // Velocity.
    public int getVelocity();
 
}

package org.wikitechy.tutorial.itf;
 
// This interface defines a standard, 
// about things capable of drinking.
public interface CanDrink {
 
    // Fields in Interface are always 'public static final'.
    // Whether you declare it or not.
    public static final String PEPSI = "PEPSI";
    final String NUMBER_ONE = "NUMBER ONE";
    String SEVENUP = "SEVEN UP";
 
    public void drink();
 
}

package org.wikitechy.tutorial.itf;
 
// This interface defines a standard, 
// about things capable of eating.
public interface CanEat {
 
    public void eat();
}

package org.wikitechy.tutorial.cls;
 
import org.wikitechy.tutorial.itf.CanMove;
 
// This class extends from Object.
// And implements CanMove interface.
// CanMove has 3 abstract methods.
// This class implements only one abstract method of CanMove.
// Therefore it must be declared as abstract.
// The remaining abstract methods to be implemented in the subclasses.
public abstract class Animal implements CanMove {
 
    // Implements run() method of CanMove.
    @Override
    public void run() {
        System.out.println("Animal run...");
    }
 
}

package org.wikitechy.tutorial.cls;
 
import org.wikitechy.tutorial.itf.CanDrink;
import org.wikitechy.tutorial.itf.CanEat;
 
// This class extends Animal and implements CanEat, CanDrink interfaces.
// This class is not declared as abstract.
// So it must implements all abstract methods of the interfaces.
public class Cat extends Animal implements CanEat, CanDrink {
 
    private String name;
 
    public Cat(String name) {
        this.name = name;
    }
 
    public String getName() {
        return this.name;
    }
 
    // Implements method of CanMove
    @Override
    public void back() {
        System.out.println(name + " cat back ...");
    }
 
    // Implements method of CanMove.
    @Override
    public int getVelocity() {
        return 110;
    }
 
    // Implements method of CanEat.
    @Override
    public void eat() {
        System.out.println(name + " cat eat ...");
    }
 
    // Implements method of CanDrink.
    @Override
    public void drink() {
        System.out.println(name + " cat drink ...");
    }
 
}

package org.wikitechy.tutorial.cls;
 
import org.wikitechy.tutorial.itf.CanDrink;
import org.wikitechy.tutorial.itf.CanEat;
 
public class Mouse extends Animal  implements CanEat, CanDrink {
 
  @Override
  public void back() {
      System.out.println("Mouse back ...");
  }
 
  @Override
  public int getVelocity() {
      return 85;
  }
 
  @Override
  public void drink() {
      System.out.println("Mouse drink ...");
  }
 
  @Override
  public void eat() {
      System.out.println("Mouse eat ...");
  }
 
}

package org.wikitechy.tutorial.cls;
 
import org.wikitechy.tutorial.itf.CanEat;
 
public class AnimalDemo {
 
    public static void main(String[] args) {
 
        // Inherit static field from CanDrink interface.
        System.out.println("Drink " + Cat.SEVENUP);
 
        // Create CanEat object
        // via constructor of Cat.
        CanEat canEat1 = new Cat("Tom");
 
        // An object declared as CanEat.
        // But in fact is Mouse.
        CanEat canEat2 = new Mouse();
 
        // Polymorphism shown here.
        // Java know the actual types of objects.
        // ==> Tom cat eat ...
        canEat1.eat();
        // ==> Mouse eat ...
        canEat2.eat();
 
        boolean isCat = canEat1 instanceof Cat;
 
        System.out.println("catEat1 is Cat? " + isCat);
 
        // Cast
        if (canEat2 instanceof Mouse) {
            Mouse mouse = (Mouse) canEat2;
 
            // Call drink method (Inherited from CanDrink).
            mouse.drink();
        }
 
    }
}