In Java, the word "String" is not a keyword. That is, we can write:
String String = "";
without the compiling error.
Also, java.lang.String class can not be used as a second parameter of the "instanceof" operator.
Why? What's so special about the String class for the Java compiler and why it isn`t a keyword?
First, String is a Class name, not a keyword. So you are free to use it as a variable name, although it's not recommended.
Second, I don't think your statement on "java.lang.String class can not be used as a second parameter of the "instanceof" operator" is correct.
I have created a test program named T2.java, it compiles and runs as expected:
import java.util.*;
import java.util.regex.*;
import java.lang.reflect.*;
public class T2 {
public static void main(String[] args) throws Exception {
String String = "abc";
outputAll((String instanceof java.lang.String), String);
}
public static void outputAll(Object... args) {
System.out.println();
for (Object o: args) {
System.out.println(o);
}
}
public static void output(String msg, Object... args) {
System.out.println(String.format(msg, args));
}
}
Of course you can use String as second parameter of instanceof. There is nothing special about the String class. The second parameter of instanceof wants a Class and the first parameter wants an expression (a variable is a valid expression).
This works for me (tried it):
String String = "";
if(String instanceof String) {
System.out.println("hallo");
}
The String class can be used as the class parameter of the instanceof operator. My guess is that if you have:
String String = ""
if(String instanceof String)
// ...
The compiler sees String on the right hand side of instanceof and thinks it isn't a class name since you defined a variable named String in that scope.
Try renaming your variable & see what happens.
Edit: After seeing alexvetter's answer, I realize I (in the above) am wrong. String instanceof String works fine for me.
String isn't a keyword because it's simply a normal class. Keywords are generally reserved for things that control the language & that - if used as variables - would confuse meaning & make for difficulty in parsing. You can use a String in all of the same ways you can use any other object in Java.
SocketorFile, etc.