The paper "Decompiling Java Bytecode: Problems, Traps and Pitfalls" contains this small but important test program (concatenation of 4 Java source files):

public class Circle implements Drawable {
    public int radius;
    public Circle(int r) {radius = r;}
    public boolean isFat() {return false;}
    public void draw() {
        // Code to draw...
    }
}

public class Rectangle implements Drawable {
    public short height, width;
    public Rectangle(short h, short w) {
        height = h; width = w; }
    public boolean isFat() {return (width > height);} 
    public void draw() {
        // Code to draw ...
    }
}
public interface Drawable {
    public void draw();
}

public class Main {

    public static void f(short i) {
        Circle c; Rectangle r; Drawable d;
        boolean is_fat;

        if (i > 10) {                   // 6
            r = new Rectangle(i, i);    // 7
            is_fat = r.isFat();         // 8
            d = r;                      // 9
        }
        else {
            c = new Circle(i);          // 12
            is_fat = c.isFat();         // 13
            d = c;                      // 14
        }
        if (!is_fat) d.draw();          // 16
    }                                   // 17

    public static void main(String args[])
        { f((short) 11); }
}

The tricky thing about this program is the assignment to the local variable d, which is of type Drawable. This really tests Java decompilers, since the types of local variables is one of the two main problems that they have.

-- MikeVanEmmerik - 13 Feb 2003