编译错误“不兼容的类型”

Question

I have compile error:

Error: incompatible types: Object cannot be converted to String.

at line String buf = it.next();

public String getMostFrequentColor() {
    HashMap<String, Integer> colors = countColors();
    int count = 0;
    String mfcolour;
    Iterator it = colors.keySet().iterator();
    while (it.hasNext()) {
        String buf = it.next();
        if (colors.get(buf) > count) {
            count = colors.get(buf);
            mfcolour = buf;
        }
    }

    return mfcolour;
}

I don't have any Idea why this could happen. it.next() should return a String in my opinion.

Answer 1

使用Iterator<String>代替Iterator 。

Iterator<String> it = colors.keySet().iterator();

Answer 2

You are using an Iterator without a generic argument. This means that it will return Object types. Either amend its declaration (by turning Iterator it into Iterator<String> it ) or manually cast the object retrieved by it.next() .
The latter may be subject to type-safety issues though!

Answer 3

The return type of the next() method in the Iterator class is Object . Since you know that your HashMap has a key set of type String , you'll need to cast the result of it.next() to String :

String buf = (String) it.next();

Answer 4

Try casting String to prevent this problem at compile time. The compiler gives you this warning simply because Java is a strict-typed language. At runtime, if the variable cannot be cast, you will only run into problems then.

String buf = (String) it.next();

Or you could make it more specific by specifying the type of Iterator you want to use. This is probably the better option.

Iterator<String> it = colors.keySet().iterator();