Question

我可以创建一个仅使用=运算符实例化的类，就像String类一样吗？或者这是Java中String类特有的功能吗？

Answer 1

No, you can't create a class that's instantiated just with = operator because you can't overload an operator in Java like you can in C++ or C# (see Operator overloading in Java).

Strings are instantiated when you use "something" only if it does not already exist in the memory, so you get a reference to the same exact String object each time you write "something".

For example, if you do:

String a = "something";
String b = "something";

Then

a == b; // will be true.

You can take a look at this questions to learn more about how String objects work:

Answer 2

由于Java不支持用户定义的运算符重载，因此无法使用=运算符创建新实例。

查看Why doesn't Java offer operator overloading?了解详情

Answer 3

代码String s = "Hello World!"不会创建新的String。它将字符串池中存在的String引用分配给s。如果字符串池中不存在String，则会在字符串池中创建新的String对象，但不能单独使用运算符=。

这会创建新的String objecs：

String s1 = new String("Hello World!"); // new Object
String s2 = new String("Hello World!"); // new Object

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

这可能会也可能不会在String Pool中创建一个新的String对象：

String s1 = "Hello World!";
String s2 = "Hello World!";

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

使用getInstance()模式可以非常接近上述行为，请考虑以下事项：

public class Singleton {
  private Singleton(){}

  private static class SingletonHelper{
    private static final instance INSTANCE = new Singleton();
  }

  public static Singleton getInstance() {
    return SingletonHelper.INSTANCE;
  }
}

然后你可以使用：

Singleton s = Singleton.getInstance();

创建不可变对象，在没有new的情况下实例化

3 个答案: