从 Arraylist 返回对象 [英] Return Objects from Arraylist

问题描述

我的问题是，我想从 ArrayList块"返回一个对象.我的代码不起作用 - 错误提示 This method must return a result of type block

My problem is, that I want to return an Object from the ArrayList "blocks". My code doesn't work - error says This method must return a result of type block

public block getBlockUnderneath (int x, int y){
    for(int i = 0; i<blocks.size(); i++){
         if (blocks.get(i).x == x) {
             return blocks.get(i);
         }
    }
}

推荐答案

你有两个问题:

1. 如果 blocks.size()==0 你的方法什么都不返回
2. 如果 blocks 中的块都没有 block.x==x，则您的方法不返回任何内容.

1. If blocks.size()==0 your method returns nothing
2. If none of the blocks in blocks have block.x==x your method returns nothing.

在 Java 中，方法必须返回它的值，声明这样做.

In Java a method must return a value of it is declared to do so.

解决您问题的最简单方法是在方法末尾return null:

The easiest solution to your issue is to return null at the end of the method:

public block getBlockUnderneath (int x, int y){
    for(final block b : blocks){
         if (b.x == x) {
             return b;
         }
    }
    return null;
}

请注意，这使用了增强的 for 循环，这是在 Java 中循环Collections(或任何实现 Iterable)的推荐方法.

Notice this uses an enhanced-for-loop, this is the recommended way to loop over Collections (or anything that implements Iterable<T>) in Java.

更好的方法可能是在未找到任何项目时抛出异常:

A better approach might be to throw an exception if no item is found:

public block getBlockUnderneath (int x, int y){
    for(final block b : blocks){
         if (b.x == x) {
             return b;
         }
    }
    throw new NoSuchElementException();
}

无论哪种情况，您都需要在调用此方法的代码中处理极端情况.

In either case you would need to handle the corner case in code that calls this method.

附言请遵守 Java 命名约定.类应该在 PascalCase 中 - 所以你的 block 类应该被称为 Block.

P.S. please stick to Java naming conventions. Classes should be in PascalCase - so you block class should be called Block.

只是为了好玩，在 Java 8 中:

Just for fun, in Java 8:

public block getBlockUnderneath(int x, int y) {
    return blocks.stream().filter((b) -> b.x == x).findFirst().get();
}

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