Java8 Streams - 使用 Stream Distinct 删除重复项

Java8 Streams - Remove Duplicates With Stream Distinct(Java8 Streams - 使用 Stream Distinct 删除重复项)

问题描述

我有一个流，例如:

Arrays.stream(new String[]{"matt", "jason", "michael"});

我想删除以相同字母开头的名称，以便只剩下一个以该字母开头的名称(不管是哪个).

I would like to remove names that begin with the same letter so that only one name (doesn't matter which) beginning with that letter is left.

我试图了解 distinct() 方法有效.我在文档中读到它基于对象的equals"方法.但是，当我尝试包装 String 时，我注意到 equals 方法从未被调用，并且没有任何内容被删除.我这里有什么遗漏吗?

I'm trying to understand how the distinct() method works. I read in the documentation that it's based on the "equals" method of an object. However, when I try wrapping the String, I notice that the equals method is never called and nothing is removed. Is there something I'm missing here?

包装类:

static class Wrp {
    String test;
    Wrp(String s){
        this.test = s;
    }
    @Override
    public boolean equals(Object other){
        return this.test.charAt(0) == ((Wrp) other).test.charAt(0);
    }
}

还有一些简单的代码:

public static void main(String[] args) {
    Arrays.stream(new String[]{"matt", "jason", "michael"})
    .map(Wrp::new)
    .distinct()
    .map(wrp -> wrp.test)
    .forEach(System.out::println);
}

推荐答案

当你重写 equals 时，你还需要重写 hashCode() 方法，这将是用于distinct()的实现.

Whenever you override equals, you also need to override the hashCode() method, which will be used in the implementation of distinct().

在这种情况下，您可以使用

In this case, you could just use

@Override public int hashCode() {
   return test.charAt(0);
}

...这样就可以了.

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