General Java Questions §

Core Java Features §

• Object oriented
• Inheritance
• Encapsulation
• Polymorphism
• Abstraction
• Platform independent
• High performance
• Multi-threaded
• Pass-by-value

Primitives §

Because of these primitive types, Java is not strictly object-oriented:

• byte
• string
• int
• float
• char
• double
• boolean
• long

These often get wrapped in Wrapper classes, which convert primitives to reference types (objects).

Volatile Keyword §

In Java, each thread has its own stack including its own copy of any variables it can access. When the thread is created, it copies the value of all accessible variables into the stack.

The volatile keyword tells the JVM that this variable might be modified in another thread - so the thread is forced to read it directly from memory rather than using the cached value.

Classloader §

Part of the runtime environment that loads classes on demand (lazily loading) into the JVM.

1. Bootstrap classloader - loads core Java API files
2. Extension classloader - loads JAR files from folder
3. Application classloader - loads JAR files from path specified in CLASSPATH env variable

String Pool §

String string = "Hello"
String string = new String("Hello")

First option is more efficient, because a String with value Hello will be created in the String pool. If another string with the same value is created, it will reference the same object.

Calling new String("...") creates a String with value Hello in the String pool, and that string is then passed to the String constructor. This creates another String object which is not in the String pool. Each call therefore creates a new object rather than just reusing an object from the pool.