Your first MiniJava program

MiniJava is Java with the parts that complicate a compiler removed — no interfaces, no overloading, no static members beyond main. What is left is small enough to hold in your head and still large enough to have objects, inheritance and a heap.

The starter skeleton

A fresh workspace is not empty. It contains the smallest well-formed program, which is also the shape every MiniJava program has:

class MainClassName {
  public static void main(String[] args) {
    System.out.println(0);
  }
}

On the workspace this is a Goal block — the grammar's start symbol — holding a main class, with a second socket labelled Class waiting for your own class declarations. A MiniJava program is exactly one main class plus any number of other classes, and the Goal block is that rule made visible.

Your first program

Change what main prints:

1. Click the MainClassName field and give the class a name.
2. From Values, drag an Integer block into the System.out.println socket, replacing the 0.
3. To compute something instead, drop an Arithmetic block from Expressions into that socket and fill its two operands.
4. Press ▶ Run.

Local variables go in the method's variables socket as Variable Declaration blocks, each taking a type and a name. MiniJava requires all declarations before any statements, and the block layout enforces that by giving them their own socket rather than letting you interleave them.

Adding a class

Drag a Class Declaration from the Program category into the Class socket under the main class. A class declaration has four sockets:

A Method Declaration in turn takes a return type, a name, a stack of Formal Parameter blocks, local variables, statements, and a return value expression. Every MiniJava method returns a value — there is no void except on main.

Instances come from the new Object block in Values; call a method with Method Call from Expressions, which takes the receiver in its in object socket and a stack of Argument Item blocks.

Running it

▶ Run in the workspace title bar executes the program to completion under the faithful MiniJava semantics and puts anything printed on the Inspector's Output tab.

For anything beyond the final answer, use the Semantics dock. That is where the program stops being a thing that produces a number and starts being a thing you can watch execute.

The code editor

The Inspector's Code tab shows the generated MiniJava with syntax highlighting — and it is editable. Type Java-ish text, and B-MJ parses it back into blocks.

This is genuinely useful for larger programs: writing a nested loop as text is faster than assembling it, and once it is blocks you can step it. The parser is the same one the round-trip tests exercise — text → blocks → text is checked to be stable.

Reading the status bar

• File name — Project.java by default, or the .bml you loaded.
• Blocks — the block count. The starter skeleton is 4; the Aliasing Contrast example is 41.
• Problems — the type-checker diagnostic count, mirroring the Problems tab.
• Autosave — Autosave ready, or Autosaved with a time.