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In java, int, byte and boolean are primitives, while String is not.

This can be contrasted with c where string is a primitive (essentially an array of chars).

What is the term for types that aren't primitive?

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The most general term (language agnostic) is a composite data type.

In Java there are several types of composite data type, the most obvious is a Class but an Enum would be another example.

A boxed type is another related term, which is a primitive wrapped in a class to provide an object oriented interface. Examples of this in Java are an int vs. an Integer or a char vs. a Character.

In java, int, byte and boolean are primitives, while String is not.

A String is actually a boxed type. Under the hood it's implemented as a char array just like C and you could use a char array too if you wanted. Obviously most people don't do this as you won't have any of the convenient abstractions provided by the String class.

Here's how the java.lang.String class is implemented in openjdk:

/** The value is used for character storage. */
private final char value[];

As you can see, there's nothing special about a string in Java.

  • Depending on the language, there may be abstract non-primitive datatypes that aren't composite (the implementation uses a primitive type, but the type system ensures you can't rely on that.) Additionally, wrapped primitive types are only "composite" in a rather degenerate way. The best way to describe the opposite is primitive is probably just "non-primitive", or perhaps "user-defined". – Doval Apr 9 '15 at 11:39
  • I don't think you can consider String a boxed type, since the underlying character array is actually a reference type. In C# though you can consider it a boxes type, I believe, because it wraps a pointer to a character array. – Sebazzz Apr 15 '15 at 15:07

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