Java SE 8 Programmer I (OCAJP 8): Java Basics

Java Basics

This domain checks whether you can read, compile, and run a small Java program and reason about declarations and scope. Expect questions on the JDK/JRE/JVM relationship, the main method, packages and imports, comments, and where variables live.

Toolchain and source structure

javac compiles .java source into .class bytecode; java starts the JVM and runs a class.
JDK = development tools + JRE; JRE = JVM + core libraries. The JVM itself is platform-dependent, while bytecode is portable.
A public top-level class must match its file name. Package declaration comes first, then imports, then type declarations.
import java.util.*; imports types in that package only, not subpackages.

Entry point and scope

A runnable class needs public static void main(String[] args). Modifier order may vary and String... args is legal, but the method must be static, public, void, and named main. Local variables are block-scoped and have no default value; instance and static fields do. Static members belong to the class, instance members to an object.

Worked example

public class Scope {
  static int count;
  public static void main(String[] args) {
    int x = 1;
    if (x > 0) {
      int y = 2;
      x = y;
    }
    System.out.println(y);
  }
}

y is declared inside the if block, so the final line does not compile. count would print 0 because static fields receive default values.

Traps

A local variable used before assignment is a compile error, not a runtime error.
System.gc() only suggests collection; it does not guarantee it.
Comments do not affect scope, but a comment cannot rescue a misplaced declaration.
A non-public class may have a different file name, but only the public class can be launched by that name.