java核心技术----Object类

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