Illustrated Guide to Core Java Concepts

1. String Immutability

The diagram shows what happens when the following code runs:

String s = "abcd";
s = s.concat("ef");

2. Difference Between equals() and hashCode()

hashCode is designed to improve performance. The key 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

The red part of the diagram represents checked exceptions, which must be either caught or declared in the method signature.

4. Collection Framework Hierarchy

Note the difference between Collections (a utility class with static methods) and Collection (the root interface of the hierarchy).

5. Java Synchronization

The synchronization mechanism is illustrated with an analogy to a building.

6. Aliasing

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

7. Heap and Stack

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

8. Java Virtual Machine Runtime Data Areas

The diagram illustrates the entire JVM runtime data area layout.