Master Java Core Concepts: From String Immutability to JVM Memory Layout

One picture is worth a thousand words; the diagrams below are taken from Program Creek's Java tutorial and have received the most votes. Use the titles to quickly grasp each concept.

1. String Immutability

The diagram shows what the following code does: String s = "abcd";<br/>s = s.concat("ef"); 2. Difference Between equals() and hashCode()

hashCode is designed to improve performance. The differences are:

If two objects are equal, they must have the same hash value.

If two objects have the same hash value, they are not necessarily equal.

3. Java Exception Class Hierarchy

Red parts in the diagram indicate checked exceptions, which must be caught or declared.

4. Collection Class Hierarchy

Note the difference between Collections and Collection: Collections provides static utility methods for collection operations.

5. Java Synchronization

The synchronization mechanism can be explained by analogy to a building.

6. Aliasing

Aliasing means multiple variables refer to the same mutable memory block, each representing different object types.

7. Heap and Stack

The diagram shows where methods and objects reside in runtime memory.

8. Java Virtual Machine Runtime Data Area

The diagram displays the entire JVM runtime data area.