testInherits.java

Address.java

public final class Address
{
    private final String detail;
    private final String postCode;

    //在構造方法裡初始化兩個例項屬性
    public Address()
    {
        this.detail = "";
        this.postCode = "";

    }
    public Address(String detail , String postCode)
    {
        
 
this.detail = detail;
        this.postCode = postCode;
    }
    //僅為兩個例項屬性提供getter方法
    public String getDetail()
    {
         return this.detail;
    }

    public String getPostCode()
    {
         return this.postCode;
    }
    //重寫equals方法，判斷兩個物件是否相等。
    public boolean equals(Object obj)
    {
        
 
if (obj instanceof Address)
        {
            Address ad = (Address)obj;
            if (this.getDetail().equals(ad.getDetail()) && this.getPostCode().equals(ad.getPostCode()))
            {
                return true;
            }
        }
        return false;
    }
    public int hashCode()
    {
        
 
return detail.hashCode() + postCode.hashCode();
    }
}

ExplorationJDKSource.java

public class ExplorationJDKSource {

    /**
     * @param args
     */
    public static void main(String[] args) {
        System.out.println(new A());
    }

}

class A{}

結果分析

main方法實際上呼叫的是：

public void println(Object x)，這一方法內部呼叫了String類的valueOf方法。

valueOf方法內部又呼叫Object.toString方法：

public String toString() {

    return getClass().getName() +"@" +

    Integer.toHexString(hashCode());

}

hashCode方法是本地方法，由JVM設計者實現：

public  native int hashCode()

Fruit.java

public class Fruit
{
        
    public String toString()
    {
        return "Fruit toString.";
    }

    public static void main(String args[])
    {
        Fruit f=new Fruit();
        System.out.println("f="+f);
    //    System.out.println("f="+f.toString());
    }
}

TestInstanceof.java。

TestInstanceof.java
public class TestInstanceof
{
    public static void main(String[] args) 
    {
        //宣告hello時使用Object類，則hello的編譯型別是Object，Object是所有類的父類
        //但hello變數的實際型別是String
        Object hello = "Hello";
        //String是Object類的子類，所以返回true。
        System.out.println("字串是否是Object類的例項：" + (hello instanceof Object));
        //返回true。
        System.out.println("字串是否是String類的例項：" + (hello instanceof String));
        //返回false。
        System.out.println("字串是否是Math類的例項：" + (hello instanceof Math));
        //String實現了Comparable介面，所以返回true。
        System.out.println("字串是否是Comparable介面的例項：" + (hello instanceof Comparable));
        String a = "Hello";
        //String類既不是Math類，也不是Math類的父類，所以下面程式碼編譯無法通過
        //System.out.println("字串是否是Math類的例項：" + (a instanceof Math));
    }
}

TestCast.java

class Mammal{}
class Dog extends Mammal {}
class Cat extends Mammal{}

public class TestCast
{
    public static void main(String args[])
    {
        Mammal m;
        Dog d=new Dog();
        Cat c=new Cat();
        m=d;
        //d=m;
        d=(Dog)m;
        //d=c;
        //c=(Cat)m;

    }
}

第二個和第四個會引起編譯出錯

ParentChildTest.java

public class ParentChildTest {
    public static void main(String[] args) {
        Parent parent=new Parent();
        parent.printValue();
        Child child=new Child();
        child.printValue();
        
        parent=child;
        parent.printValue();
        
        parent.myValue++;
        parent.printValue();
        
        ((Child)parent).myValue++;
        parent.printValue();
        
    }
}

class Parent{
    public int myValue=100;
    public void printValue() {
        System.out.println("Parent.printValue(),myValue="+myValue);
    }
}
class Child extends Parent{
    public int myValue=200;
    public void printValue() {
        System.out.println("Child.printValue(),myValue="+myValue);
    }
}

TestPolymorphism.java

class Parent
        
{

    public int value=100;
    
public void Introduce()
    {

            System.out.println("I'm father");

        }


}

class Son extends Parent
{

    public int value=101;

         public void Introduce()
    {

            System.out.println("I'm son");
    
}

}


class Daughter extends Parent
{

    public int value=102;
    public void Introduce()
    {

            System.out.println("I'm daughter");
    
}

}

public class TestPolymorphism
{


    public static void main(String args[])
    {

        Parent p=new Parent();

        p.Introduce();

        System.out.println(p.value);

        p=new Son();

        p.Introduce();

        System.out.println(p.value);

        p=new Daughter();    

        p.Introduce();

        System.out.println(p.value);


    }


}