Java中ArrayLists的getter和Setter

Question

How would I go about using getters and setters for an ArrayList in Java? Here is what I have. I think I am using the getter correctly, but I don't know how to use a setter for an ArrayList.

private ArrayList<String> stringlist = new ArrayList<String>();
public ArrayList<String> getStringList() {
    return stringlist;
}

public ArrayList<String> setStringList() {
    // I don't know what to put here
    // I also don't know if my return value is correct
}

Answer 1

ArrayList is a mutable container class, though, so you don't actually need a setter at all: simply have callers call getStringList() and then mutate the ArrayList themselves:

public final class DataHolder {
  private final ArrayList<String> stringList = new ArrayList<>();

  public ArrayList<String> getStringList() {
    return stringList;
  }
}

public static void main(String[] args) {
  DataHolder data = new DataHolder();
  data.getStringList().add("foo");
}

On the other hand, the more common requirement is to prevent callers from being able to mutate internal data structures, so that your class can actually enforce its invariants on its data. ArrayList is always mutable, so if you need to return an ArrayList but you don't want your private state to be modified, you'll need to copy:

public ArrayList<String> getStringList() {
  return new ArrayList<String>(stringList);
}

Alternatively, if possible, it's better to widen the return type and then return some other implementation that your class can use to enforce its invariants. For example, if you don't want people to modify your ArrayList using a getter method, you could widen the type from ArrayList<String> to List<String> and use an unmodifiable implementation, for example:

public List<String> getStringList() {
  return Collections.unmodifiableList(stringList);
}

On the other hand, for the setter, it depends on how you want to handle the values. Normally a setFoo method replaces the contents of foo, which you can of course do:

public void setStringList(ArrayList<String> stringList) {
  this.stringList = stringList;
}

But most likely you want to also widen the type you will accept. For example, you could accept any Collection instead of just an ArrayList :

public void setStringList(Collection<String> strings) {
  this.stringList = new ArrayList<>(strings);
}

It may be more useful, however, to instead expose methods to mutate the underlying list in other ways. For example, perhaps you simply want to support adding new items to the list:

public void addString(String string) {
  this.stringList.add(string);
}

public void addStrings(Collection<String> strings) {
  this.stringList.addAll(strings);
}

If you do decide to expose a setter that replaces the value, you probably want to first check it for correctness. Since your class initializes the ArrayList in an instance initializer, most likely you don't expect it will ever be null. So you should throw an exception if it is:

public void setStringList(List<String> stringList) {
  if (stringList == null) {
    throw new NullPointerException("stringList must not be null");
  }
  this.stringList = stringList;
}

Java 7 added a new Objects.requireNonNull method for exactly this purpose.

Answer 2

A typical setter would look like this:

public void setStringList(ArrayList<String> stringList) {
    this.stringList = stringList;
}

You can also consider returning this for purpose of fluent coding .

public TypeOfYourClass setStringList(ArrayList<String> stringList) {
    this.stringList = stringList;
    return this;
}

Answer 3

To set the value, you would take an ArrayList<String> passed as a parameter, and you would simply set the ArrayList to the parameter.

Also, note the use of void rather than the ArrayList<String> shown in your question. Commonly, a setter method does not return anything.

public void setStringList(ArrayList<String> stringList)
{
    this.stringList = stringList;
}

Answer 4

Instead setter function, you can use a constructor to assign array. like:

public class CLL extends ArrayAdapter<LBean> {
  ArrayList<LBean> lbean;

  public CLL(Context context, ArrayList<LBean> lList) {
      super(context, R.layout.custom_layoutT, lList );
      this.lbean=llist;
  }
  //getter functions
}

Answer 5

One of the simplest things we as programmers do is pass around data. The traditional way to do this is to define a JavaBean:

public class DataHolder {
private String data;

public DataHolder() {
}

public void setData(String data) {
    this.data = data;
}

public String getData() {
    return this.data;
}
}

Instead, I prefer the C struct style of writing classes that merely hold data:

public class DataHolder {
public final String data;

public DataHolder(String data) {
    this.data = data;
}
}