Simply description some part of essence of Java This one involves String & enum

1. Java Essence part1
- String, enum
Bram

2. Outline
 String
 enum

3. String
 String str = "Java";
 String str = new String("Java");
 String temp = "Java";
String str = new String(temp);
 String str1 = "Java";
String str2 = new String(str1);
System.out.println(str1 == str2); // false
 Str1 is not the same object of str2

4. String
 String (Intern) Pool
 preserving immutable strings in pool
 one instance of the same string
 String str1 = "Java";
String str2 = "Java";
System.out.println(str1 == str2); // true
 String str1 = "a" + "bc";
String str2 = "ab" + "c";
System.out.println(str1 == str2); // true

5. String
 String.intern()
 If pool has this string, return it directly
otherwise, add to pool and return
 s.intern() == t.intern() is true
if and only if s.equals(t) is true
 String str1 = "Java";
String str2 = new String(str1);
System.out.println(str1 == str2.intern()); //true

6. String
 String.intern()
 Use only if needing to compare a lot of String
 Improve by == to replace equals()
 no way to de-intern
 In general,
not to call intern() on user-generated strings
 In JDK 7
 interned strings aren‟t allocated in the permanent
generation, but in the main part of the Java heap

7. String
 String.substring()
 Instead of creating an new char array,
just „reuse‟ the old one
 Only the offset and count changes
 public final class String{
private final char value[];
private final int offset;
private final int count;
public String substring(int beginIndex, int endIndex)
{
return new String (offset + beginIndex,
endIndex - beginIndex, value);}
}
}

8. enum
 Without Enum
 public interface Position {
public static final int TOP = 0;
public static final int RIGHT = 1;
public static final int LEFT = 2;
public static final int BOTTOM = 4;
}
 Valuables in Interface
 public : open for others to understand
 static : there is no instances, so it must belong to Class
 final : regards Interface as treaty
– Add by compiler even not declare those key-words

9. enum
 After JDK5
 enum is available
 public enum Position {
TOP, RIGHT, LEFT, BOTTOM
}
 “enum” declare special class
 Inherit from java.lang.Enum
– CANNOT extent directly

10. enum
 After Decompile
 public final class Position extends Enum {
…….
private Position(String s, int i) {
super(s, i);
}
public static final Position TOP;
public static final Position RIGHT;
public static final Position LEFT;
public static final Position BOTTOM;
...
static {
TOP = new Position("TOP", 0);
RIGHT = new Position("RIGHT", 1);
LEFT = new Position("LEFT", 2);
UP = new Position("UP", 3);
...
}
}

11. enum
 java.lang.Enum
 public abstract class Enum<E extends Enum<E>>
implements Comparable<E>, Serializable {
…..
private final String name;
private final int ordinal;
protected Enum(String name, int ordinal) {
this.name = name;
this.ordinal = ordinal;
} // ordinal is the enum order with beginning of 0
…
}
 http://grepcode.com/file/repository.grepcode.com/java/root/jdk/
openjdk/7-b147/java/lang/Enum.java

12. enum
 Extended enum
 public enum MethodSet {
INIT (“init”, -1),
START(“start”, 1),
STOP(“stop”, 0);
….
private string method;
private int value;
private MethodSet(string method, int value) {
this.methid = method;
this.value = value;
}
public string method() {
return method;
}
public int value() {
return value;
}
}

13. enum
 After decompile
 public final class MethodSet extends Enum {
...
private MethodSet(String s, int i, string method, int value) {
super(s, i);
this.methid = method;
this.value = value;
}
public int value() {
return value;
}
...
public static final MethodSet INIT;
public static final MethodSet START;
public static final MethodSet STOP;
private string method;
private int value;
private static final MethodSet $VALUES[];

14. enum
 continue.
 static
{
INIT = new MethodSet("INIT", 0, "init", -1);
START = new MethodSet("START", 1, "start", 1);
STOP = new MethodSet("STOP", 2, "stop", 0);
$VALUES = (new MethodSet[] {
INIT, START, STOP
});
}
}

15. enum
 More Extended enum, v1
 public enum Action implements Command {
STOP, RIGHT, LEFT;
public void execute() {
switch(this) {
case STOP:
System.out.println(“Animation Stop");
break;
case RIGHT:
System.out.println(“Animation Right");
break;
case LEFT:
System.out.println(“Animation Left");
break;
}
}
}

16. enum
 More Extended enum, v2
 public enum Action implements Command {
STOP {
public void execute() {
System.out.println(“Animation Stop");
}
},
RIGHT {
public void execute() {
System.out.println(“Animation Right");
}
},
LEFT {
public void execute() {
System.out.println(“Animation Left");
}
};
}

17. enum
 After decompile
 public abstract class Action extends Enum implements Command {
...
static
{
STOP = new Action("STOP", 0) {
public void execute() {
System.out.println("u64AD…xxxxx");
}
};
RIGHT = new Action("STOP", 1) {
public void execute() {
System.out.println("u64AD…xxxxx");
}
};
...
}
...
}

18. Java Decompiler
 http://java.decompiler.free.fr/
 JD-GUI
– http://java.decompiler.free.fr/?q=jdgui
 JD-Eclipse
 JD-Core