/**
* Indicates whether some other object is "equal to" this one.
* <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
* {@code x}, {@code x.equals(x)} should return
* {@code true}.
* <li>It is <i>symmetric</i>: for any non-null reference values
* {@code x} and {@code y}, {@code x.equals(y)}
* should return {@code true} if and only if
* {@code y.equals(x)} returns {@code true}.
* <li>It is <i>transitive</i>: for any non-null reference values
* {@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}.
* <li>It is <i>consistent</i>: for any non-null reference values
* {@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}.
* </ul>
* <p>
* 该方法用于识别两个对象之间的相似性
* 也就是说，对于一个非null值，x和y，当且仅当它们指向同一个对象时才会返回true
* 言外之意，和==没啥两样。
* 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}).
* <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.
*
* @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
*/
public boolean equals(Object obj) {
return (this == obj);
}

package com.eq;

import java.io.InputStream;

public class DefaultEq {
String name;
public DefaultEq(String name){
this.name = name;
}
public static void main(String[] args) {
DefaultEq eq1 = new DefaultEq("张三");
DefaultEq eq2 = new DefaultEq("张三");
DefaultEq eq3 = eq1;

//虽然俩对象外面看起来一样，eq和==都不行
//因为我们没有改写equals，它使用默认object的，也就是内存地址
System.out.println(eq1.equals(eq2));
System.out.println(eq1 == eq2);

System.out.println("----");
//1和3是同一个引用
System.out.println(eq1.equals(eq3));
System.out.println(eq1 == eq3);

System.out.println("===");
//以上是对象，再来看基本类型
int i1 = 1;
Integer i2 = 1;
Integer i = new Integer(1);
Integer j = new Integer(1);

Integer k = new Integer(2);

//只要是基本类型，不管值还是包装成对象，都是直接比较大小
System.out.println(i.equals(i1)); //比较的是值
System.out.println(i==i1); //拆箱 ,
// 封装对象i被拆箱，变为值比较，1==1成立
//相当于 System.out.println(1==1);

System.out.println(i.equals(j)); //
System.out.println(i==j); // 比较的是地址，这是俩对象

System.out.println(i2 == i); // i2在常量池里，i在堆里，地址不一样

System.out.println(i.equals(k)); //1和2，不解释
}
}

public static void main(String[] args) {
User user1=new User("张三");
User user2=new User("张三");
}

public boolean equals(Object obj) {
return (this == obj);//本类比较的是内存地址(引用)
}

public static void main(String[] args) {
//需求：：user1和user2，在现实生活中是一个人；如何判定是一个人（相等）
User user1 = new User("张三");
User user2 = new User("张三");
System.out.println("是否同一个人："+user1.equals(user2));
System.out.println("内存地址相等："+String.valueOf(user1 == user2));//内存地址
System.out.println("user1的hashCode为>>>>" + user1.hashCode());
System.out.println("user2的hashCode为>>>>" + user2.hashCode());
}

public static void main(String[] args) {
String str1 = "张三";//常量池
String str2 = new String("张三");//堆中
String str3 = new String("张三");//堆中
System.out.println("是否同一人："+str1.equals(str2));//这个地方为什么相等呢，重写
System.out.println("是否同一人："+str2.equals(str3));//这个地方为什么相等呢，重写
//如果相等，hashcode必须相等，重写
System.out.println("str1的hashCode为>>" + str1.hashCode());
System.out.println("str2的hashCode为>>" + str2.hashCode());
}
}

/**
* Compares this string to the specified object. The result is {@code
* true} if and only if the argument is not {@code null} and is a {@code
* String} object that represents the same sequence of characters as this
* object.
*
* @param anObject
* The object to compare this {@code String} against
*
* @return {@code true} if the given object represents a {@code String}
* equivalent to this string, {@code false} otherwise
*
* @see #compareTo(String)
* @see #equalsIgnoreCase(String)
*/
public boolean equals(Object anObject) {
//如果内存地址相等，那必须equal
if (this == anObject) {
return true;
}
if (anObject instanceof String) {
//如果对象是String类型
String anotherString = (String)anObject;
int n = value.length;
if (n == anotherString.value.length) {
//并且长度还相等！
char v1[] = value;
char v2[] = anotherString.value;
int i = 0;
//那我们就逐个字符的比较
while (n-- != 0) {
//从前往后，任意一个字符不匹配，直接返回false
if (v1[i] != v2[i])
return false;
i++;
}
//全部匹配结束，返回true
return true;
}
}
return false;
}

@Override
public boolean equals(Object o) {
//注意这些额外的判断类操作
if (this == o) return true;
if (o == null || getClass() != o.getClass()) return false;

User user = (User) o;
//比较值
return name != null ? name.equals(user.name) : user.name == null;
}

@Override
public int hashCode() {
//返回值的hashCode
return name != null ? name.hashCode() : 0;
}

package com.eq;

import java.util.HashSet;
import java.util.Set;

public class Contains {
public static void main(String[] args) {
User user1 = new User("张三");
User user2 = new User("张三");
Set set = new HashSet();
set.add(user1);
System.out.println(set.contains(user2));


User2 user3 = new User2("张三");
User2 user4 = new User2("张三");
Set set2 = new HashSet();
set2.add(user3);
System.out.println(set2.contains(user4));
}
}

public class Intern {
public static void main(String[] args) {
String str1 = "张三";//常量池
String str2 = new String("张三");//堆中

//intern；内存地址是否相等(面试常问)
System.out.println("str1与str2是否相等>>" +(str1==str2)); // false
System.out.println("str1与str2是否相等>>" +(str1==str2.intern())); // true

}
}

package com.eq;

public class Valueof {
public static void main(String[] args) {
System.out.println( Integer.valueOf(127) == Integer.valueOf(127));
System.out.println( Integer.valueOf(128) == Integer.valueOf(128));
}
}

/**
* Returns an {@code Integer} instance representing the specified
* {@code int} value. If a new {@code Integer} instance is not
* required, this method should generally be used in preference to
* the constructor {@link #Integer(int)}, as this method is likely
* to yield significantly better space and time performance by
* caching frequently requested values.
*
* ！在-128 到 127 之间会被cache，同一个地址下，超出后返回new对象！
*
* This method will always cache values in the range -128 to 127,
* inclusive, and may cache other values outside of this range.
*
* @param i an {@code int} value.
* @return an {@code Integer} instance representing {@code i}.
* @since 1.5
*/
public static Integer valueOf(int i) {
if (i >= IntegerCache.low && i <= IntegerCache.high)
return IntegerCache.cache[i + (-IntegerCache.low)];
return new Integer(i);
}