Java Best Practices – Is It Bad Practice to Create New Objects Without Storing Them?

java

I've seen objects created in Java code without storing a reference to the object. For example, in an eclipse plugin I've seen a SWT Shell created like so:

new Shell();

This new Shell object is not stored in a variable, but will remain referenced until the window is disposed which [I believe?] happens by default when the window is closed.

Is it bad practice to create objects like this without storing a reference to them? Or was the library poorly designed? What if I don't have need of a reference, but only want the "side effects" of the object? Should I store a reference anyways?

UPDATE:

Admitedly, my above example is poor. While I have seen UI elements created like this, creating a SWT Shell like this would probably be pointless because you need to call the open method on the Shell instance. There are better examples provided by aix such as the following from the Java concurrency tutorial:

(new HelloThread()).start();

This practice is seen in many contexts, so the questions remains. Is it good practice?

Best Answer

There's an element of personal preference to this, but I think that not storing the reference is not necessarily a bad practice.

Consider the following hypothetical example:

new SingleFileProcessor().process(file);

If a new processor object needs to be created for every file, and is not needed after the process() call, there's no point in storing a reference to it.

Here is another example, taken from the Java concurrency tutorial:

(new HelloThread()).start();

I've seen lots of other examples when the reference is not stored, and that read perfectly fine to my eye, such as:

String str = new StringBuilder().append(x).append(y).append(z).toString();

(The StringBuilder object is not kept.)

There are similar patterns involving common.lang's HashCodeBuilder et al.

Related Solutions

Java – Overriding – Access to Members with Reference Reassignment

If you look at the Java Language Specification (http://docs.oracle.com/javase/specs/) you will find a distinction between the static type and the dynamic type of object references.

The static type of any reference refers to the type declared in your program at development time.

The dynamic type of the reference refers to the type of object which is actually stored at the reference address at runtime.

So if you have a simple hierarchy, where A is the superclass and B is a subclass of A.

class A {
  public String a = "super";
  void m() {...}
}

class B extends A {
  public String a = "sub";
  @Override
  void m() {...}
}

A refA = new A();
A refB = new B();
B refB2 = new B();
refA.m();
refB.m();
System.out.println(refA.a);
System.out.println(refB.a);
System.out.println(refB2.a);

For example, refB's static type is A but its dynamic type is B. What does this imply at runtime?

Instance variables are statically bound, i.e. you can only access the instance variables of the static type, so the first two println() statements will print "super", since A is the static type of refA and refB. The last println() statement will in contrast print "sub", because in this case B is the static type.
Methods are dynamically bound, i.e. the invoked method is first searched in the dynamic type of the reference and if it isn't found, the class hierarchy is traversed up until a definition of the overriden method is found (this mechanism is called dynamic dispatch). This means that in case refA.m() method void m() of A is invoked and in case refB.m(), void m() of B is invoked.

Java – Keeping track of all objects of a class

I don't know why you need to keep a list of all instances of a class.

That would cause memory leakage since those objects will never be disposed of, since the list will still be referencing them after no other class does.

But if you really want to follow that route:

Use the Factory pattern. A factory class with methods that instanciate the class and return the objects. That way you have a centralize point to control the instantiations.
Use the Singleton pattern to hold a list or lists that hold the instances.
Make the factory put each object of a certain type in a list after creating them.

By the way: constructors are inherited.

Best Answer

Related Solutions

Java – Overriding – Access to Members with Reference Reassignment

Java – Keeping track of all objects of a class

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