This note will provide a quick overview of different out of memory errors you might enounter, how to identify the type of the error, how you can analyze them further and how you can try to avoid them in future. We will also see some JVM flags which can help us in this analysis.

We will see the four main cases of out of memory error:

1. Not enough native memory for JVM

2. Not enough permgen/metaspace memory in heap

3. Not enough heap memory

There are different kinds of references in Java like strong, soft, weak, phantom etc. When you create a normal object using the new keyword and then assign it to a regular type, you are creating a strong reference. Garbage collector will only collect an object if there are no strong references to that object. Less strong references include soft, weak and phantom, and they are defined in the java.lang.ref package.

String interning is a method of storing only one copy of each distinct string value. Strings in Java are immutable and hence this sharing is perfectly safe and give you better performance. The distinct values are stored in a fixed-size hashtable usually referred to as string intern pool or string pool. The single copy of each string is called its 'intern'. You can read more about the basics of String interning with examples @ string-interning-in-java-with-examples.