为什么返回的数组对于Java String.split（）具有不同的长度？

Question

String.split（）在Java中到底做什么？为什么情况2和情况3的长度不同？

案例1：

String line = "a,b,c,d";
String[] parts = line.split(",");
System.out.println("Length:" + parts.length);

Length: 4

案例2：

String line = "a,,,d";
String[] parts = line.split(",");
System.out.println(parts.length);

Length: 4

案例3：

String line = "a,b,,";
String[] parts = line.split(",");
System.out.println(parts.length);

Length: 2