英文:
Why arrayList.contains() returns false for this usecase where as System.out.print() writes same on console?
问题
为什么 arrayList.contains() 对于这个用例返回 false，而 System.out.print() 在控制台上输出相同的结果？
ArrayList<Float> random = new ArrayList<Float>();
random.add(2f);
random.add(4f);
random.add(5f);
random.add(10f);
random.add(99.9f);
System.out.println(random.contains(5.0)); -- 这会返回 false
然而，
System.out.println(random.get(2)); -- 这会返回 5.0
英文:
Why arrayList.contains() returns false for this usecase where as System.out.print() writes same on console ?
ArrayList<Float> random = new ArrayList<Float>();
random.add(2f);
random.add(4f);
random.add(5f);
random.add(10f);
random.add(99.9f);
System.out.println(random.contains(5.0)); -- this returns false
Where as,
System.out.println(random.get(2)); -- this returns 5.0
答案1
得分: 1
在Java中，一个字面的小数是一个double，即5.0是一个double。
contains()方法期望一个Object（而不是一个基本数据类型，比如double），所以Java会将double自动装箱成一个Double包装对象，所以你的测试就好像你编写了这样的代码：
System.out.println(random.contains(new Double(5.0))); -- 总是返回false
因为它们是不同的类型，一个Double永远不会等于一个Float，所以它总是会返回false。
然而，你可以编写一个字面的float，在字面值后面添加一个F（或f）。
试试这个：
System.out.println(random.contains(5.0F));
英文:
In Java, a literal decimal is a double, ie 5.0 is a double.
The contains() method expects an Object (not a primitive, such as double), so Java autoboxes the double to a Double wrapper object, so your test is as if you have coded this:
System.out.println(random.contains(new Double(5.0))); -- always false
Because they are different types, a Double is never going to be equal to a Float, so it's always going to be false.
However, you can code a literal float, add an F (or an f) after the literal.
Try this:
System.out.println(random.contains(5.0F));
答案2
得分: 0
我认为这是因为您正在比较两个浮点数与双精度数，5.0 是一个双精度数。但也可能是两个不同的引用。在这种情况下，这个讨论可能会对您有所帮助：https://stackoverflow.com/questions/18852059/java-list-containsobject-with-field-value-equal-to-x
英文:
I think this is because you are comparing two floats with doubles, 5.0 is a double. But could also be two different references. In which case this discussion may help you: https://stackoverflow.com/questions/18852059/java-list-containsobject-with-field-value-equal-to-x
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