Java中null、isEmpty、""的区别

判断对象是否为空：
str == null

判断值是否为空：
str.isEmpty()

public class Test {

    public void test() {
        String a = null;
        String b = new String();
        String c = "";
        if (a == null) {
            System.out.println("a == null : true");
        } else {
            System.out.println("a == null : false");
        }
        if (b == null) {
            System.out.println("b == null : true");
        } else {
            System.out.println("b == null : false");
        }
        if (c == null) {
            System.out.println("c == null : true");
        } else {
            System.out.println("c == null : false");
        }
        if (b.isEmpty()) {
            System.out.println("b.isEmpty() : true");
        } else {
            System.out.println("b.isEmpty() : false");
        }
        if (c.isEmpty()) {
            System.out.println("c.isEmpty() : true");
        } else {
            System.out.println("c.isEmpty() : false");
        }
        if (b.equals("")) {
            System.out.println("b.equals(\"\") : true");
        } else {
            System.out.println("b.equals(\"\") : false");
        }
        if (c.equals("")) {
            System.out.println("c.equals(\"\") : true");
        } else {
            System.out.println("c.equals(\"\") : false");
        }
        if (b == "") {
            System.out.println("b == \"\" : true");
        } else {
            System.out.println("b == \"\" : false");
        }
        if (c == "") {
            System.out.println("c == \"\" : true");
        } else {
            System.out.println("c == \"\" : false");
        }
    }
}

输出
a == null : true
b == null : false
c == null : false
b.isEmpty() : true
c.isEmpty() : true
b.equals("") : true
c.equals("") : true
b == "" : false
c == "" : true