The difference between == and equals in Java, a simple explanation, very clear

What is the difference between

==" and the equals method?

(Explain one thing clearly separately, and then explain the other clearly. In this way, the difference will naturally come out. If they are mixed together, it will be difficult to explain clearly)

= The = operator is specially used to compare whether the values ​​of two variables are equal, that is, to compare whether the values ​​stored in the memory corresponding to the variables are the same. To compare whether two basic types of data or two reference variables are equal, just You can use the == operator.

If the data pointed to by a variable is of object type, then two pieces of memory are involved. The object itself occupies one piece of memory (heap memory), and the variable also occupies a piece of memory, such as Objet obj = newObject(); The variable obj is a memory, and new Object() is another memory. At this time, the value stored in the memory corresponding to the variable obj is the first address of the memory occupied by the object. For variables pointing to the object type, If you want to compare whether two variables point to the same object, that is, to see whether the values ​​in the memory corresponding to the two variables are equal, you need to use the == operator for comparison. The

equals method is used to compare the two variables. Whether the contents of two independent objects are the same is like comparing whether two people have the same appearance. The two objects compared are independent. For example, for the following code:

String a=new String("foo");

String b=new String("foo");

The two new statements create two objects, and then use the two variables a/b to point to one of the objects respectively. These are two different objects. Their The first addresses are different, that is, the values ​​stored in a and b are different, so the expression a==b will return false, and the contents of the two objects are the same, so the expression a.equals (b) will return true.

In actual development, we often need to compare whether the passed string content is equal, for example, String input = ...;input.equals("quit"), many people do this without paying attention. It is wrong to use == for comparison. Just look at some practical teaching videos on the Internet and you will find a lot of such errors. Remember, string comparisons basically use the equals method

.

If a class does not define its own equals method, then it will inherit the equals method of the Object class. The implementation code of the equals method of the Object class is as follows:

boolean equals(Object o){

return this==o;

}

This shows that if a class does not define its own equals method, its default equals method (inherited from the Object class) is to use the == operator, which is also comparing whether the objects pointed to by the two variables are the same object. At this time, use equals will get the same result as using ==, and will always return false if two independent objects are compared. If the class you write hopes to be able to compare whether the contents of two instance objects created by the class are the same, then you must override the equals method and write your own code to determine under what circumstances the contents of the two objects are considered to be the same.

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Simply put, == compares whether two people are really the same person. equals is generally used to compare whether two people are logically equal (such as stipulations If two people have the same height as adults, they will be the same, etc.), you can define it however you want. If you don't override the equals method, the default is still to compare whether the two people are the same person (nonsense, both people are still in the embryonic state, there is no Specific characteristics, how can it be logically compared).

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