1. Building a Java Interpreter
CS 142 (b)
04/10/18

2. Lab Sessions Monday, Wednesday
10am – noon
ICS 189
Send an or make an appointment with the TA

3. Java Interpreter Input: a bytecode instruction stream
Output: a native-code instruction stream
In our project:
Translate each bytecode instruction to C++ code

4. Instructions We Are Interested in
iconst, iload and istore
iadd
if_icmpne, if_icmpeq, if_icmpgt, if_icmplt, ifeq, ifne, ifgt, iflt
goto
invokestatic
return

5. Building an Interpreter
void interpret(InstructionSeq is){ for (Instruction i = is.first(); i != null; i = next) { int opCode = i.getOpCode(); next = is.nextInstruction(i); switch(opCode){ case iload: int index = i.getOperand(); int value = values.get(index); push(value); break; case istore: int value = pop(); values.set(index, value); }

6. Building an Interpreter (Cond.)
for(Instruction i = is.first(); i != null; i = next){
int opCode = i.getOpCode(); next = is.nextInstruction(i);
switch(opCode){
case iadd:
int value1 = pop(); int value2 = pop();
push(value1 + value2);
break;
case if_icmpeq:
int value1 = pop(); int value2 = pop(); int target = i.getOperand();
if(value1 == value2) next = is.getInstruction(target);
case goto:
int target = i.getOperand();
next = is.getInstruction(target); }

7. Building an Interpreter (Cond.)
void interpret(InstructionSeq is){ for(Instruction i = is.first(); i != null; i = next){ … case invokestatic: String mName = ((CallInsruction) i). getTarget(); InstructionSet newSet = findInstructionSetforMethod(mName); invoke(newSet); break; }

8. Handling of println
If the target of a static method is System.out.println, just call C++ function printf on the parameter string