Java Compile and Execution Flow

This chapter answers the first interview-style questions you collected:

• What is the role of compiler?
• What happens in Java compilation phase?
• What happens in execution phase?

1. Role of Java Compiler (javac)

Java compiler (javac) converts human-readable Java source code (.java) into platform-independent bytecode (.class).

What compiler does

1. Syntax checking
2. Type checking
3. Semantic checking (class/method/variable references)
4. Bytecode generation

2. What Happens in Compilation Phase?

When you run:

javac Test.java

compiler performs:

1. Syntax Validation

   • Verifies Java grammar, keyword usage, braces, method signatures, class declarations.
   • Java is case-sensitive, so identifiers and keywords must match exact case.

2. Type Validation (Strong Typing)

   • Ensures assigned values match declared data types.
   • Example: assigning text to an int is rejected at compile time.

3. Comment Ignoring

   • Comments are skipped by compiler and do not affect bytecode output.

4. Bytecode Generation

   • Produces .class file containing JVM bytecode.

3. What Happens in Execution Phase?

When you run:

java Test

JVM performs:

1. Bytecode Loading via Class Loader.
2. Bytecode Verification for security and correctness.
3. Memory Allocation for class metadata, objects, stacks, etc.
4. Execution by Interpreter + JIT Compiler.

4. End-to-End Example

Test.java --(javac)--> Test.class --(java + JVM)--> Output

• javac creates bytecode.
• JVM loads and verifies bytecode.
• Execution engine runs code and prints output.

5. Quick Interview Answer

Q: What is compiler role in Java?
Compiler checks code correctness and converts .java source file into .class bytecode for JVM execution.

Q: What happens in compilation phase?
Syntax check, type check, comment ignoring, bytecode generation.

Q: What happens in execution phase?
Class loading, bytecode verification, memory setup, and execution using interpreter/JIT.