如何在Android中保存全局“列表”或至少将其实例化以用于任何活动？

Question

I create a class called Singleton which contains a list and instance

public class  Singleton {
private static Singleton instance;
private List<MaterialModel> list = new ArrayList<MaterialModel>();

public Singleton(){
}

public List<MaterialModel> getList() {
    return list;
}

public void setList(List<MaterialModel> list) {
    this.list = list;
}

public static synchronized Singleton getInstance(){
    if(instance == null){
        instance = new Singleton();
    }
    return instance;
}

public void addElement(MaterialModel material){
    this.list.add(material);
}

public MaterialModel getElement(int pos){
    return this.list.get(pos);
}

public boolean foundElement(MaterialModel material){
    if(this.list.contains(material)){
        return true;
    }
    return false;
}


}

The idea is to store items on the list and get them in any activity. In reality I'm trying to get this list can be used in any kind and do not lose items within it.

First activity

public class FirstActivity extends SherlockActivity{
    Singleton singleton;
        ...
       public onCreate(...){
       singleton.addElement(obj)); // MaterialModel obj = new MaterialModel(.....)

Second Activty

public class SecondActivity extends SherlockActivity{
        Singleton singleton;
            ...
           public onCreate(...){
           MaterialModel tmp = singleton.getElement(0)); // getElement return "obj"

The problem is that does not work, how can I fix it or at least save the list globaly so that it can be used in any activity ?

Answer 1

You need to get the instance first of your Singleton before modifying any data in it.

example:

every time you call the Singleton singleton you need to call the static instance of it and instantiate in your Singleton object;

Singleton singleton = Singleton.getInstance();

Answer 2

I don't think you understand the idea behind the 'singleton' pattern.

Firstly, don't create a public constructor - instead (as Rod_Algonquin) suggests you need to use a static method (usually called getInstance ) to return the single instance of the singleton.

Example...

public class Singleton {
    private static Singleton mInstance = null;

    public static Singleton getInstance() {
        if (mInstance == null)
            mInstance = new Singleton();
        return mInstance;
    }

    // Other code here

}

In the above code the singleton class holds a reference to a single instance of itself as mInstance . To start with mInstance is null and as there is no public constructor, it's not possible to create a new instance of the singleton from another class by using new Singleton() but it is possible from within the singleton itself.

Instead, whenever you need to access your singleon use...

Singleton.getInstance();

If you look at the code for getInstance() you'll see...

if (mInstance == null)
    mInstance = new Singleton();
return mInstance;

Obviously the first time getInstance() is called mInstance will be null so the if condition will be true and the line mInstance = new Singleton() will be executed. Following that, the method will return the instance referenced by mInstance .

On any subsequent calls to Singleton.getInstance() from any class or method in your code, mInstance will already be instantiated and therefore the if condition will be false but the getInstance method will still return mInstance as the single instance of the singleton.

As an added bonus you can either hold a reference to the singleton in your code and call a method such as...

Singleton theSingleton = Singleton.getInstance();
theSingleton.doSomething();

...or you can simply use it to chain a call to a public method in the singleton such as...

Singleton.getInstance().doSomething();

So for your first Activity you can do this...

Singleton.getInstance().addElement(obj);

...and for your second Activity do this...

MaterialModel tmp = Singleton.getInstance().getElement(0);