java - “.equals”和“==”有什么区别？

Question

我今天换了讲师，他用一个奇怪的代码对我说。（他说最好用.equals，当我问为什么时，他回答“因为它是！”）

所以这里有一个例子：

if (o1.equals(o2))
{
 System.out.println("Both integer objects are the same");
}

而不是我习惯的：

if (o1 == o2)
{
  System.out.println("Both integer objects are the same");
}

两者有什么区别。为什么他的方式（使用.equals）更好？

在快速搜索中找到了这个，但我无法理解这个答案：

Answer 1

在Java 中，==总是只比较两个引用（即对于非原始对象）——即它测试两个操作数是否引用同一个对象。

但是，该equals方法可以被覆盖——因此两个不同的对象仍然可以相等。

例如：

String x = "hello";
String y = new String(new char[] { 'h', 'e', 'l', 'l', 'o' });

System.out.println(x == y); // false
System.out.println(x.equals(y)); // true

此外，值得注意的是，任何两个相等的字符串常量（主要是字符串文字，但也包括通过串联的字符串常量组合）最终将引用同一个字符串。例如：

String x = "hello";
String y = "he" + "llo";
System.out.println(x == y); // true!

这里x和y是对同一个字符串的引用，因为y是一个编译时常量，等于"hello".

Answer 2

== 运算符比较对象是否是相同的实例。equals() 操作符比较对象的状态（例如，如果所有属性都相等）。当一个对象等于另一个对象时，您甚至可以重写 equals() 方法来定义自己。

Answer 3

如果你我每个人走进银行，每个人都开一个全新的账户，每个人都存入 100 美元，那么……

1. myAccount.equals(yourAccount)是true因为它们具有相同的值，但是
2. myAccount == yourAccount是false因为他们不是同一个账号。

（当然，假设类的适当定义Account。;-)

Answer 4

== 是一个运算符。equals 是 Object 类中定义的方法

== 检查两个对象在内存中是否具有相同的地址，对于原语，它检查它们是否具有相同的值。equals 方法另一方面检查正在比较的两个对象是否具有相等的值（取决于当然已经为对象实现了 equals 方法。equals 方法不能应用于基元（这意味着如果 a 是基元 a.equals(someobject) 是不允许的，但是 someobject.equals(a) 是允许的）。

Answer 5

== operator compares two object references to check whether they refer to same instance. This also, will return true on successful match.for example

public class Example{
public static void main(String[] args){
String s1 = "Java";
String s2 = "Java";
String s3 = new string ("Java");
test(Sl == s2)     //true
test(s1 == s3)      //false
}}

above example == is a reference comparison i.e. both objects point to the same memory location

String equals() is evaluates to the comparison of values in the objects.

   public class EqualsExample1{
   public static void main(String args[]){
   String s = "Hell";
   String s1 =new string( "Hello");
   String s2 =new string( "Hello");
   s1.equals(s2);    //true
    s.equals(s1) ;   //false
    }}

above example It compares the content of the strings. It will return true if string matches, else returns false.

Answer 6

Answer 7

public static void main(String[] args){
        String s1 = new String("hello");
        String s2 = new String("hello");

        System.out.println(s1.equals(s2));
        ////
        System.out.println(s1 == s2);

    System.out.println("-----------------------------");

        String s3 = "hello";
        String s4 = "hello";

        System.out.println(s3.equals(s4));
        ////
        System.out.println(s3 == s4);
    }

Here in this code u can campare the both '==' and '.equals'

here .equals is used to compare the reference objects and '==' is used to compare state of objects..

Answer 8

Answer 9

Lets say that "==" operator returns true if both both operands belong to same object but when it will return true as we can't assign a single object multiple values

public static void main(String [] args){
    String s1 = "Hello";
    String s1 = "Hello";  // This is not possible to assign multiple values to single object
    if(s1 == s1){
      // Now this retruns true
   }
}

Now when this happens practically speaking, If its not happen then why this is == compares functionality....

Answer 10

(1) == can be be applied for both primitives and object types, but equals() method can be applied for only object types.

(2) == cannot be overridden for content comparison, but equals method can be overridden for content comparison(ex; String class, wrapper classes, collection classes).

(3) == gives incomparable types error when try to apply for heterogeneous types , where as equals method returns false.

Answer 11

Here is a simple interpretation about your problem:

== (equal to) used to evaluate arithmetic expression

where as

equals() method used to compare string

Therefore, it its better to use == for numeric operations & equals() method for String related operations. So, for comparison of objects the equals() method would be right choice.