从集合中获取元素 [英] Getting an element from a Set

问题描述

为什么不设置一个操作来获取等于另一个元素的元素？

 设置< Foo> set = ...; 
 ... 
 Foo foo = new Foo（1，2，3）; 
 Foo bar = set.get（foo）; //从Set等于foo获取Foo元素
  

我可以询问Set是否包含元素等于bar，为什么我不能得到那个元素？ ：（

为了说明， equals 方法被覆盖，但它只检查其中一个字段， 。所以两个被认为相等的Foo对象实际上可能有不同的值，这就是为什么我不能使用 foo 。

如果相等，就没有必要获得元素。 Map 更适合这个用例。 / p>

---

如果您仍然想要查找元素，则没有其他选项，只能使用迭代器：

  public static void main（String [] args）{
 
 Set< Foo> set = new HashSet< Foo> ; 
 set.add（new Foo（Hello））; 
 
 for（Iterator< Foo> it = set.iterator（）; it.hasNext（）;）{
 Foo f = it.next（）; 
 if（f.equals（new Foo（Hello）））
 System.out.println（foo found）; 
 } 
} 
 
 static class Foo {
 String string; 
 Foo（String string）{
 this.string = string; 
} 
 @Override 
 public int hashCode（）{
 return string.hashCode（）; 
} 
 @Override 
 public boolean equals（Object obj）{
 return string.equals（（（Foo）obj）.string）; 
} 
} 
  

Why doesn't Set povide an operation to get an element that equals another element?

Set<Foo> set = ...;
...
Foo foo = new Foo(1, 2, 3);
Foo bar = set.get(foo);   // get the Foo element from the Set that equals foo

I can ask whether the Set contains an element equal to bar, so why can't I get that element? :(

To clarify, the equals method is overriden, but it only checks one of the fields, not all. So two Foo objects that are considered equal can actually have different values, that's why I can't just use foo.

解决方案

There would be no point of getting the element if it is equal. A Map is better suited for this usecase.

---

If you still want to find the element you have no other option but to use the iterator:

public static void main(String[] args) {

    Set<Foo> set = new HashSet<Foo>();
    set.add(new Foo("Hello"));

    for (Iterator<Foo> it = set.iterator(); it.hasNext(); ) {
        Foo f = it.next();
        if (f.equals(new Foo("Hello")))
            System.out.println("foo found");
    }
}

static class Foo {
    String string;
    Foo(String string) {
        this.string = string;
    }
    @Override
    public int hashCode() { 
        return string.hashCode(); 
    }
    @Override
    public boolean equals(Object obj) {
        return string.equals(((Foo) obj).string);
    }
}

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