Points to remember about Java Clone -

• Avoid cloning an object whenever possible. Prefer copy constructor and static factory instance method whenever possible.
• A class which wants to clone should implement “Cloneable” interface. This interface doesn’t contain any methods. Then it should override “clone” method present in the Object class. Implementation of it should always invoke super class clone method and add any additional attribute cloning of its own class. If you don’t clone the attributes of its class any modification to this class attributes will lead to modifying the original object’s attributes.
• If a super class doesn’t implement clone properly its impossible to inherit from that class and implement clone.

<script type=”text/javascript”>
function toggle() {
var elm = document.getElementById(‘abc’);
elm.removeAttribute(‘readonly’);
alert(‘In toggle’);
}
</script>

<button onclick=”toggle();”>Click Me!!</button>
<textarea id=”abc” readonly>
Some content
</textarea>

Then I ran into issue of removeAttribute was not working in IE. After some (re)search found out that IE has an optional case insensitivity flag to removeAttribute.  Replacing the above “removeAtrribute” call by -

removeAttribute(“abc”, 0) worked like charm. The same code works well in Safari as well as FireFox.

How to solve this? What we did is that, store last modification time of the file whenever we found a valid record. Whenever we found a null record, compare current files’ modification time against last modification time we have stored. If its newer and we are still getting null means file is rotated.

package com.test;

import java.util.HashMap;
import java.util.Map;

public class EqualsTest {

String s;

public EqualsTest(String s1) {
s = s1;
}

public boolean equals(Object obj) {
EqualsTest o = (EqualsTest)obj;
return s.equals(o.s);
}

public static void main(String[] args) {
EqualsTest eq = new EqualsTest(“ABC”);
Map<EqualsTest, Integer> m = new HashMap<EqualsTest, Integer>();
m.put(eq, 1);

EqualsTest eq1 = new EqualsTest(“ABC”);
if(m.containsKey(eq1)) {
System.out.println(“Eq1 present in map…”);
} else {
System.out.println(“Eq1 is not present in map…”);
}
}
}

Add hashCode method to EqualsTest class to get correct ouput of – “Eq1 present in map…”.

public int hashCode() {
return s.hashCode();
}

• Its done automatically by JVM. Less error prone.
• Java compiler recongnizes constrctors as special functions which can initialize final members of the class. If constructor calls a function that does initialization its not recognized by java compiler throws a compilation error of “uninitilized final member variable”.

For ex., Following code results in compilation error.

package com.test;
public class CtorInitBlockTest {
final int x;
CtorInitBlockTest() {
init();
}

private void init() {

x  = 10;
}
}

Solution to the problem would be -

package com.test;
public class CtorInitBlockTest {
final int x;
{
x = 10;
}

CtorInitBlockTest() {
}
}

For ex., user ‘X’ creates a directory dir.

$ mkdir dir

$ chmod 0777 dir

$ touch dir/file

$ ls -l dir/file

-rw-rw-r– 1 X X 0 May 6 11:39 dir/file

User Y can come delete this file since he has write permissions for the directory.

$ rm dir/file ## voilla your file gone

How do you protect this file? Sticky bit is the answer for this. When a sticky bit set on a directory, only file’s owner or directorie’s owner or super user can delete this file. Setting sticky bit is as simple as -

$ chmod +t dir

or

$ chmod 1777 dir

Most common use of this technique can be seen on /tmp directory.

package com.test;

public class ClassInitTest {
private String y = “INIT VALUE”;
private int x = init();

private static int init() {
return 10;
}

public static void main(String[] args) {
ClassInitTest ci = new ClassInitTest();
System.out.println(”x: ” + ci.x + “, y: ” + ci.y);
}

}