package java.lang;

/**

 * Class {@code Object} is the root of the class hierarchy.

 * Every class has {@code Object} as a superclass. All objects,

 * including arrays, implement the methods of this class.

 *

 * @author  unascribed

 * @see     java.lang.Class

 * @since   JDK1.0

 */

public class Object {

    private static native void registerNatives();

    static {

        registerNatives();

    }

    public final native Class<?> getClass();

    public native int hashCode();

    public boolean equals(Object obj) {

        return (this == obj);

    }

    protected native Object clone() throws CloneNotSupportedException;

    public String toString() {

        return getClass().getName() + "@" + Integer.toHexString(hashCode());

    }

    public final native void notify();

    public final native void notifyAll();

    public final native void wait(long timeout) throws InterruptedException;

    public final void wait(long timeout, int nanos) throws InterruptedException {

        if (timeout < 0) {

            throw new IllegalArgumentException("timeout value is negative");

        }

        if (nanos < 0 || nanos > 999999) {

            throw new IllegalArgumentException(

                                "nanosecond timeout value out of range");

        }

        if (nanos > 0) {

            timeout++;

        }

        wait(timeout);

    }

    public final void wait() throws InterruptedException {

        wait(0);

    }

    protected void finalize() throws Throwable { }

}

Object类是Java中所有类的始祖，在Java中每个类都是由它扩展而来。

在Java中，只有基本类型不是对象（数组也都扩展了Object类）。

/**

 * Created by N3verL4nd on 2016/12/4.

 * Object类：所有类的超类

 * Object类是不断抽取而来，具备着所有对象都具备的共性内容。

 * 常用的共性内容：

 * equals toString hashCode clone

 */

class Person{

	private int age;

	public Person(int age){

		this.age = age;

	}

}

public class test {

    public static void main(String... args) {

        Person p1 = new Person(20);

        Person p2 = new Person(20);

        System.out.println(p1 == p2);

        System.out.println(p1.equals(p2));

    }

}

输出：

false

false

equals比较的是两个对象是否指向同一个位置

重写equals方法：

/**

 * Created by N3verL4nd on 2016/12/4.

 * Object类：所有类的超类

 * Object类似不断抽取而来，具备着所有对象都具备的共性内容。

 * 常用的共性内容：

 * equals toString hashCode clone

 */

class Person{

	private int age;

	public Person(int age){

		this.age = age;

	}

	//根据Person类的年龄进行比较

	@Override

	public boolean equals(Object obj){

		if (!(obj instanceof Person)) {

			throw new ClassCastException("类型错误");

		}

		Person p = (Person)obj;

		return (age == p.age);

	}

}

public class test {

    public static void main(String... args) {

        Person p1 = new Person(20);

        Person p2 = new Person(21);

        Person p3 = new Person(20);

        System.out.println(p1 == p2);

        System.out.println(p1.equals(p2));

        System.out.println(p1.equals(p3));

    }

}

输出：

false

false

true

/**

 * Created by N3verL4nd on 2016/12/4.

 * Object类：所有类的超类

 * Object类似不断抽取而来，具备着所有对象都具备的共性内容。

 * 常用的共性内容：

 * equals toString hashCode clone

 */

class Person{

	private int age;

	public Person(int age){

		this.age = age;

	}

	//根据Person类的年龄进行比较

	@Override

	public boolean equals(Object obj){

		if (!(obj instanceof Person)) {

			throw new ClassCastException("类型错误");

		}

		Person p = (Person)obj;

		return (age == p.age);

	}

}

public class test {

    public static void main(String... args) {

        Person p1 = new Person(20);

        Person p2 = new Person(21);

        Person p3 = new Person(20);

        Demo demo = new Demo();

        System.out.println(p1 == p2);

        System.out.println(p1.equals(p2));

        System.out.println(p1.equals(p3));

        System.out.println("p1 = " + p1);

        System.out.println("p1.hashCode() = " + p1.hashCode());

        System.out.println("Integer.toHexString(p1.hashCode()) = " + Integer.toHexString(p1.hashCode()));

    }

}

输出：

false false true p1 = Person@28d93b30 p1.hashCode() = 685325104 Integer.toHexString(p1.hashCode()) = 28d93b30

一般重写equals方法后都需要重写HashCode，因为相等的两个对象必须确保hashCode相等。

/**

 * Created by N3verL4nd on 2016/12/4.

 * Object类：所有类的超类

 * Object类似不断抽取而来，具备着所有对象都具备的共性内容。

 * 常用的共性内容：

 * equals toString hashCode clone

 */

class Person{

	private int age;

	public Person(int age){

		this.age = age;

	}

	//根据Person类的年龄进行比较

	@Override

	public boolean equals(Object obj){

		if (!(obj instanceof Person)) {

			throw new ClassCastException("类型错误");

		}

		Person p = (Person)obj;

		return (age == p.age);

	}

	@Override

	public int hashCode(){

		return age;

	}

}

public class test {

    public static void main(String... args) {

        Person p1 = new Person(20);

        Person p2 = new Person(21);

        Person p3 = new Person(20);

        Demo demo = new Demo();

        System.out.println(p1 == p2);

        System.out.println(p1.equals(p2));

        System.out.println(p1.equals(p3));

        System.out.println("p1 = " + p1);

        System.out.println("p1.hashCode() = " + p1.hashCode());

        System.out.println("Integer.toHexString(p1.hashCode()) = " + Integer.toHexString(p1.hashCode()));

    }

}

/**

 * Created by N3verL4nd on 2016/12/4.

 * Object类：所有类的超类

 * Object类似不断抽取而来，具备着所有对象都具备的共性内容。

 * 常用的共性内容：

 * equals toString hashCode clone

 */

class Person{

	private int age;

	public Person(int age){

		this.age = age;

	}

	//根据Person类的年龄进行比较

	@Override

	public boolean equals(Object obj){

		if (!(obj instanceof Person)) {

			throw new ClassCastException("类型错误");

		}

		Person p = (Person)obj;

		return (age == p.age);

	}

/*

	@Override

	public int hashCode(){

		return age;

	}

*/

}

public class test {

    public static void main(String... args) {

        Person p1 = new Person(20);

       	System.out.println(p1);

       	System.out.println(p1.getClass().getName() + "@" + Integer.toHexString(p1.hashCode()));

    }

}

重写toString方法：

/**

 * Created by N3verL4nd on 2016/12/4.

 * Object类：所有类的超类

 * Object类似不断抽取而来，具备着所有对象都具备的共性内容。

 * 常用的共性内容：

 * equals toString hashCode clone

 */

class Person{

	private int age;

	public Person(int age){

		this.age = age;

	}

	//根据Person类的年龄进行比较

	@Override

	public boolean equals(Object obj){

		if (!(obj instanceof Person)) {

			throw new ClassCastException("类型错误");

		}

		Person p = (Person)obj;

		return (age == p.age);

	}

/*

	@Override

	public int hashCode(){

		return age;

	}

*/

	public String toString(){

		return "Person@@" + age;

	}

}

public class test {

    public static void main(String... args) {

        Person p1 = new Person(20);

       	System.out.println(p1);

       	System.out.println(p1.getClass().getName() + "@" + Integer.toHexString(p1.hashCode()));

    }

}

输出：

Person@@20

Person@28d93b30

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