Array objects themselves always contain either an array of primitive types or an array of object references. Yet, in the Java virtual machine, arrays are handled with special bytecodes.Īs with any other object, arrays cannot be declared as local variables only array references can. ![]() Array references can be used anywhere a reference to type Object is called for, and any method of Object can be invoked on an array. ![]() In Java, arrays are full-fledged objects, and, like any other object in a Java program, are created dynamically. In the JVM instruction set, all objects are instantiated and accessed with the same set of opcodes, except for arrays. Only after the reference has been explicitly initialized - either with a reference to an existing object or with a call to new - does the reference refer to an actual object. Upon declaration, an object reference refers to nothing. Only object references can be declared as such. The architectural separation of objects and primitive types in the JVM is reflected in the Java programming language, in which objects cannot be declared as local variables. Objects can never reside on the Java stack. Only object references and primitive types can reside on the Java stack as local variables. For example, there is an Integer class that wraps an int type with an object. If you want to use a primitive type where an Object reference is needed, you can allocate a wrapper object for the type from the java.lang package. There is no way to allocate memory for a primitive type on the heap, except as part of an object. In the Java virtual machine, memory is allocated on the garbage-collected heap only as objects. Object references and primitive types reside either on the Java stack as local variables, on the heap as instance variables of objects, or in the method area as class variables. Objects reside on the garbage-collected heap. The Java virtual machine (JVM) works with data in three forms: objects, object references, and primitive types. This month's article takes a look at the bytecodes that deal with objects and arrays. ![]() It aims to give developers a glimpse of the mechanisms that make their Java programs run. This column focuses on Java's underlying technologies. Welcome to another edition of Under The Hood.
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