“==” 比较的是两个引用在内存中指向的是不是同一对象（即同一内存空间），也就是说在内存空间中的存储位置是否一致。（引用类型）
equals方法是由Object类提供的，可以由子类来进行重写
Java 语言里的 equals方法其实是交给开发者去覆写的，让开发者自己去定义满足什么条件的两个Object是equal的。
所以我们不能单纯的说equals到底比较的是什么。你想知道一个类的equals方法是什么意思就是要去看定义。（以上是来自知乎用户的回答）
我们要理解的是equals存在的目的。
JDK源码
java.lang.Object类中的equals
//在Object类中，equals的实现方法
/**
* 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>
* 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);
}java.lang.String中重写的equals方法
/**
* 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) {
//引用类型内存地址比较
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;
}“==”与equals 在不同情况下的代码示例
String str1="hello";
String str2=new String("hello");
StringBuffer str3=new StringBuffer("hello");
StringBuffer str4=new StringBuffer("hello");
StringBuffer str5=str3;
str5.append("word");
System.out.println(str1.equals(str3)); //false
System.out.println(str1==str2); //false
System.out.println(str1.equals(str2));//true
System.out.println(str4.equals(str3));//false
System.out.println(str3.equals(str5));//true
System.out.println(str3==str5);//true以上代码中的内存结构简析：
通过代码和内存结构图结合来看就会得到结论：
“==” 比较的是两个引用在内存中指向的是不是同一对象（即同一内存空间），也就是说在内存空间中的存储位置是否一致。（引用类型）
equals方法是由Object类提供的，可以由子类来进行重写
