尝试在没有比较器的情况下添加树集时，StringBuffer 不会给出 ClassCastException

Question

I was expecting the ClassCastException when trying to run the following piece of code:

 //  TreeSet<StringBuffer> t = new TreeSet<StringBuffer>((a, b) -> - 
  // a.toString().compareTo(b.toString()));
     TreeSet<StringBuffer> t = new TreeSet<StringBuffer>();
        Function<String, StringBuffer> f = s -> new StringBuffer(s);
        t.add(f.apply("A"));
        t.add(f.apply("M"));
        t.add(f.apply("B"));
        t.add(f.apply("Z"));
        t.add(f.apply("G"));
        System.out.println(t);

I have commented the Comparator code and was trying to use without the Comparator. But I am still getting the correct output. How is this possible because StringBuffer doesn't implement Comparable interface. I am using Java 11. Was there any changes made in the later version related to this.

Answer 1

You are converting StringBuffer to String in comparator, andString class implements Comparable interface, And also as a note since from jdk-11 StringBuffer and StringBuilder also implements Comparable

(a, b) -> -a.toString().compareTo(b.toString())

But if you are on lower version below jdk-11 you will get the compile time error

The method compareTo(StringBuffer) is undefined for the type StringBuffer

Answer 2

这是可能的，因为StringBuffer 确实实现了Comparable接口。

Answer 3

API Note: StringBuffer implements Comparable but does not override equals. Thus, the natural ordering of StringBuffer is inconsistent with equals. Care should be exercised if StringBuffer objects are used as keys in a SortedMap or elements in a SortedSet. See Comparable, SortedMap, or SortedSet for more information. Since: 1.0 See Also: StringBuilder, String, Serialized Form https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/lang/StringBuffer.html#:~:text=StringBuffer%20implements%20Comparable%20but%20does,or%20elements%20in%20a%20SortedSet%20 .