使用自定义Comparator在Java中创建SortedMap

Question

I want to create a TreeMap in Java with a custom sort order. The sorted keys which are string need to be sorted according to the second character. The values are also string.

Sample map:

Za,FOO
Ab,Bar

Answer 1

You can use a custom comparator like this:

    Comparator<String> secondCharComparator = new Comparator<String>() {
        @Override public int compare(String s1, String s2) {
            return s1.substring(1, 2).compareTo(s2.substring(1, 2));
        }           
    };

Sample:

    SortedMap<String,String> map =
        new TreeMap<String,String>(secondCharComparator);
    map.put("Za", "FOO");
    map.put("Ab", "BAR");
    map.put("00", "ZERO");
    System.out.println(map); // prints "{00=ZERO, Za=FOO, Ab=BAR}"

Note that this simply assumes that the String has a character at index 1. It throws StringIndexOutOfBoundsException if it doesn't.

---

Alternatively, you can also use this comparison:

return s1.charAt(1) - s2.charAt(1);

This subtraction "trick" is broken in general, but it works fine here because the subtraction of two char will not overflow an int .

The substring and compareTo solution above is more readable, though.

See also:

• Java Integer: what is faster comparison or subtraction?

Answer 2

Assuming you don't mean Hash as in hash function or the sort...

You could easily accomplish this by creating a "wrapper" class for String and overriding the compareTo method