Memory in Java

http://www.vogella.de/articles/JavaPerformance/article.html

In the heap the Java Virtual Machine (JVM) stores all objects created by the Java application, e.g. by using the "new" operator. The Java garbage collector (gc) can logically separate the heap into different areas, so that the gc can faster identify objects which can get removed

The memory for new objects is allocated on the heap at run time. Instance variables live inside the object in which they are declared.

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Stack is where the method invocations and the the local variables are stored. If a method is called then its stack frame is put onto the top of the call stack. The stack frame holds the state of the method including which line of code is executing and the values of all local variables. The method at the top of the stack is always the current running method for that stack. Threads have their own call stack.

Runtime runtime = Runtime.getRuntime();
  // Run the garbage collector
  runtime.gc();
  // Calculate the used memory
  long memory = runtime.totalMemory() - runtime.freeMemory();