Show awareness that high-level language programs may be partially compiled and partially interpreted, such as Java (console mode)

5.2 Language Translators

Learning Objective

Show awareness that high‑level language programs may be partially compiled and partially interpreted, such as Java in console mode.

Key Concepts

• Compiler – Translates the whole source program into target code (usually machine code) before execution.
• Interpreter – Executes a program by translating each statement (or small block) on the fly.
• Hybrid (partial compilation + interpretation) – Source code is first compiled into an intermediate form, which is then interpreted or further compiled at run‑time.

Why Use a Hybrid Approach?

A hybrid approach tries to combine the speed of compiled code with the flexibility of interpreted code. Typical benefits include:

1. Platform independence – the intermediate code can run on any system that provides the appropriate runtime.
2. Faster start‑up than pure interpretation because some work is done ahead of time.
3. Dynamic features (reflection, runtime class loading) remain possible.

Java in Console Mode – A Case Study

When a Java program is executed from the command line, the following steps occur:

1. Source code written in .java files.
2. Compilation by javac produces .class files containing Java bytecode (an intermediate representation).
3. Loading of the bytecode by the Java \cdot irtual Machine (J \cdot M).
4. Just‑In‑Time (JIT) compilation – the J \cdot M may translate frequently executed bytecode sections into native machine code while the program runs.
5. Interpretation – bytecode that has not yet been JIT‑compiled is interpreted directly by the J \cdot M.

The overall process can be summarised by the following equation:

\$\text{Execution Time} = \text{Interpretation Time} + \text{JIT Compilation Time}\$

Comparison of Translation Strategies

Typical Workflow for a Java Console Application

1. Write source code in MyProgram.java.
2. Compile: javac MyProgram.java → creates MyProgram.class.
3. Run: java MyProgram → J \cdot M loads MyProgram.class.
4. J \cdot M interprets bytecode; as methods are called repeatedly, the JIT compiler may translate them to native code.
5. Program terminates; any JIT‑compiled native code is discarded.

Implications for Developers

• Performance tuning often focuses on hot‑spot methods that the JIT will optimise.
• Understanding the hybrid model helps explain why some code runs faster after a “warm‑up” period.
• Debugging can be affected: line numbers map to source, but JIT‑compiled code may obscure the exact execution path.