package com.patternObsTest;

public interface Jobservers {

String update(Jsubject subj);

}

package com.patternObsTest;

public interface Jsubject {

void attach(Jobservers[] obs);

void detach(int obs);

void noty();

}

package com.patternObsTest;

public class Observer implements Jobservers {

public String update(Jsubject j){

return "Someone wants to set the value of the attribue : length.";

}

}

Here's is an implementation of some Designs Pattern from scratch.

Code source files are available in the package `com.patternObsTest`.

__What is a Design Pattern ?__

The observer pattern is a software design pattern in which an object, called the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.

```java

// importing classes in the package

import com.patternObsTest.*;

public class test{

public static void main(String[] args[]){

Subject s = new Subject();

Observer o = new Observer();

Observer o2 = new Observer();

Observer[] arr = {o,o2};

s.attach(arr);

/*now we set the value of the attribute length contains in the Subject.

s.setLength(valLength.Val2);

//Now look at your console to see the result :p

}

}

package com.patternObsTest;

public class Subject implements Jsubject {

public Jobservers[] tab;

private int length;

public void setLength(valLEngth val){

this.length = val.getVal();

this.noty();

}

public int getLength(){ return this.length;}

@Override

public void attach(Jobservers[] obs){

this.tab = obs;

}

public void detach(int i){

this.tab[i]= new Observer();

}

public void noty(){

for(int i = 0; i < this.tab.length; i++){

System.out.println(this.tab[i].update(this));

}

}

}

package com.patternObsTest;

public enum valLength {

Val1(24),

Val2(23),

val3(45);

public int val;

valLength(int val){

this.val = val;

}

public int getVal(){

return this.val;

}

@Override

public String toString() {

return "I'm an Enum !";

}

}