Object类学习之二：equals()

public boolean equals(Object obj) {
        return (this == obj);
    }

package com.xyf.javaSE2;

/**
 * @author xyf
 * @create 2020-08-25-21:27
 */
public class Demo01 {
    public static void main(String[] args) {
        String s1=new String("xyf");
        String s2=new String("xyf");
        //String s3=new String("lwy");
        String s3="xyf";
        String s4="xyf";
        System.out.println(s1==s2);
        System.out.println("==================");
        System.out.println(s1.equals(s2));
        System.out.println("====================");
        System.out.println(s3==s4);
        System.out.println("=============");
        System.out.println(s3.equals(s4));
    }
}

false
==================
true
====================
true //这里也可知道，当使用String string = “”给字符串初始化时，这个变量保存在栈里
=============
true

    public boolean equals(Object anObject) {
        if (this == anObject) {
            return true;
        }
        if (anObject instanceof String) {
            String anotherString = (String)anObject;
            int n = value.length;
            if (n == anotherString.value.length) {
                char v1[] = value;
                char v2[] = anotherString.value;
                int i = 0;
                while (n-- != 0) {
                    if (v1[i] != v2[i])
                        return false;
                    i++;
                }
                return true;
            }
        }
        return false;
    }

    public int hashCode() {
        int h = hash;
        if (h == 0 && value.length > 0) {
            char val[] = value;

            for (int i = 0; i < value.length; i++) {
                h = 31 * h + val[i];
            }
            hash = h;
        }
        return h;
    }

/**
     * Indicates whether some other object is "equal to" this one.//显示某一个其他的对象是不是与当前对象“相等”
     * <p>
     * The {@code equals} method implements an equivalence relation
     * on non-null object references:           //
     * <ul>
     * <li>It is <i>reflexive</i>: for any non-null reference value//自反性
     *     {@code x}, {@code x.equals(x)} should return
     *     {@code true}.//对于任何非空对象 x.equals(x)都应当返回true
     * <li>It is <i>symmetric</i>: for any non-null reference values//对称性
     *     {@code x} and {@code y}, {@code x.equals(y)}
     *     should return {@code true} if and only if
     *     {@code y.equals(x)} returns {@code true}.//当且仅当 y.equals(x)返回true时，x.equals(y)返回true
     * <li>It is <i>transitive</i>: for any non-null reference values//传导性
     *     {@code x}, {@code y}, and {@code z}, if
     *     {@code x.equals(y)} returns {@code true} and
     *     {@code y.equals(z)} returns {@code true}, then
     *     {@code x.equals(z)} should return {@code true}.//如果x.equals（y）返回true,y.equals(z)返回true，那么x.equals(z)也应该返回true
     * <li>It is <i>consistent</i>: for any non-null reference values//一致性
     *     {@code x} and {@code y}, multiple invocations of
     *     {@code x.equals(y)} consistently return {@code true}
     *     or consistently return {@code false}, provided no
     *     information used in {@code equals} comparisons on the
     *     objects is modified.
     * <li>For any non-null reference value {@code x},
     *     {@code x.equals(null)} should return {@code false}.//对于任何对象x x.equals(null)返回false
     * </ul>
     * <p>
     * The {@code equals} method for class {@code Object} implements
     * the most discriminating possible equivalence relation on objects;
     * that is, for any non-null reference values {@code x} and
     * {@code y}, this method returns {@code true} if and only
     * if {@code x} and {@code y} refer to the same object
     * ({@code x == y} has the value {@code true}).    //当且仅当引用变量x与引用变量y指向同一个对象时，x==y返回true
     * <p>
     * Note that it is generally necessary to override the {@code hashCode}
     * method whenever this method is overridden, so as to maintain the
     * general contract for the {@code hashCode} method, which states
     * that equal objects must have equal hash codes.//注意不管何时重写了本方法，一定要重写hashCode()方法，以此来保证hashCode()对于相等的对象返回相等的哈希码
     *
     * @param   obj   the reference object with which to compare.
     * @return  {@code true} if this object is the same as the obj
     *          argument; {@code false} otherwise.
     * @see     #hashCode()

     * @see     java.util.HashMap
     */

The general contract of {@code hashCode} is:
     * <ul>
     * <li>Whenever it is invoked on the same object more than once during     //只要用于比较的信息没有改变，不管被调用几次，同一个对象的hashcode永远保持不变
     *     an execution of a Java application, the {@code hashCode} method
     *     must consistently return the same integer, provided no information
     *     used in {@code equals} comparisons on the object is modified.
     *     This integer need not remain consistent from one execution of an
     *     application to another execution of the same application.
     * <li>If two objects are equal according to the {@code equals(Object)} //通过 equals(Object)方法比较为相同的两个对象，那么hashcode也应当相同
     *     method, then calling the {@code hashCode} method on each of
     *     the two objects must produce the same integer result.
     * <li>It is <em>not</em> required that if two objects are unequal      // 如果两个对象，用Object原始类里的初始equals()返回false时，并不要求其hashcode也不同
     *     according to the {@link java.lang.Object#equals(java.lang.Object)}//但是，如果两个不相等的对象有两个hashcode，会提高hashtable的效率
     *     method, then calling the {@code hashCode} method on each of the
     *     two objects must produce distinct integer results.  However, the
     *     programmer should be aware that producing distinct integer results
     *     for unequal objects may improve the performance of hash tables.
     * </ul>