泰山版java开发手册的一个疑问

public class TestNull {

public static void main(String[] args) {
	String a = new String();
	String b = "";
	String c = null;
	if (a.isEmpty()) {
		System.out.println("String a = new String");
	}

	if (b.isEmpty()) {
		System.out.println("String b = \"\"");
	}

	if (c == null) {
		System.out.println("String c =null");
	}

	if (null == a) {
		System.out.println("String a =null");
	}

	if (a == "") {
		System.out.println("a = ''");
	}
}

}