A brief analysis of the problem of judging whether a string String is empty in java

/**
 * Created with IntelliJ IDEA.
 * User: Administrator
 * Date: 14-1-16
 * Time: 上午10:43
 * 判断字符串是否为空的效率问题
 */
public class JudgeStringIsEmptyOrNot {
    public static void main(String[] args) {
        JudgeString1("w_basketboy", 10000);
        JudgeString2("w_basketboy", 10000);
        JudgeString3("w_basketboy", 10000);
        JudgeString4("w_basketboy", 10000);
    }
    /**
     * 方法一: 最多人使用的一个方法, 直观, 方便, 但效率很低;
     * 方法二: 比较字符串长度, 效率高, 是最好的一个方法;
     * 方法三: Java SE 6.0 才开始提供的方法, 效率和方法二几乎相等, 但出于兼容性考虑, 推荐使用方法二;
     * 方法四: 这是一种比较直观,简便的方法,而且效率也非常的高,与方法二、三的效率差不多;
     */
    public static void JudgeString1(String str, long num) {
        long startTiem = System.currentTimeMillis();
        for (int i = 0; i < num; i++) {
            for (int j = 0; j < num; j++) {
                if (str == null || "".equals(str)) {
                }
            }
        }
        long endTime = System.currentTimeMillis();
        System.out.println("function1耗时：" + (endTime - startTiem) + "ms");
    }
    public static void JudgeString2(String str, long num) {
        long startTiem = System.currentTimeMillis();
        for (int i = 0; i < num; i++) {
            for (int j = 0; j < num; j++) {
                if (str == null || str.length() <= 0) {
                }
            }
        }
        long endTime = System.currentTimeMillis();
        System.out.println("function4耗时：" + (endTime - startTiem) + "ms");
    }
    public static void JudgeString3(String str, long num) {
        long startTiem = System.currentTimeMillis();
        for (int i = 0; i < num; i++) {
            for (int j = 0; j < num; j++) {
                if (str == null || str.isEmpty()) {
                }
            }
        }
        long endTime = System.currentTimeMillis();
        System.out.println("function3耗时：" + (endTime - startTiem) + "ms");
    }
    public static void JudgeString4(String str, long num) {
        long startTiem = System.currentTimeMillis();
        for (int i = 0; i < num; i++) {
            for (int j = 0; j < num; j++) {
                if (str == null || str == "") {
                }
            }
        }
        long endTime = System.currentTimeMillis();
        System.out.println("function4耗时：" + (endTime - startTiem) + "ms");
    }
}