#vulnerable code 
                                public void atBinExpr(BinExpr expr) throws CompileError {
        int token = expr.getOperator();
        int k = CodeGen.lookupBinOp(token);
        if (k >= 0) {
            /* arithmetic operators: +, -, *, /, %, |, ^, &, <<, >>, >>>
             */
            if (token == '+') {
                Expr e = atPlusExpr(expr);
                if (e != null) {
                    /* String concatenation has been translated into
                     * an expression using StringBuffer.
                     */
                    e = CallExpr.makeCall(Expr.make('.', e,
                                            new Member("toString")), null);
                    expr.setLeft(e);
                    expr.setOprand2(null);    // <---- look at this!
                    className = jvmJavaLangString;
                }
            }
            else {
                expr.oprand1().accept(this);
                int type1 = exprType;
                expr.oprand2().accept(this);
                computeBinExprType(expr, token, type1);
            }
        }
        else {
            /* equation: &&, ||, ==, !=, <=, >=, <, >
            */
            booleanExpr(expr);
        }
    } 
                            #location 23 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                public void atBinExpr(BinExpr expr) throws CompileError {
        int token = expr.getOperator();
        int k = CodeGen.lookupBinOp(token);
        if (k >= 0) {
            /* arithmetic operators: +, -, *, /, %, |, ^, &, <<, >>, >>>
             */
            if (token == '+') {
                Expr e = atPlusExpr(expr);
                if (e != null) {
                    /* String concatenation has been translated into
                     * an expression using StringBuffer.
                     */
                    e = CallExpr.makeCall(Expr.make('.', e,
                                            new Member("toString")), null);
                    expr.setOprand1(e);
                    expr.setOprand2(null);    // <---- look at this!
                    className = jvmJavaLangString;
                }
            }
            else {
                ASTree left = expr.oprand1();
                ASTree right = expr.oprand2();
                left.accept(this);
                int type1 = exprType;
                right.accept(this);
                if (!isConstant(expr, token, left, right))
                    computeBinExprType(expr, token, type1);
            }
        }
        else {
            /* equation: &&, ||, ==, !=, <=, >=, <, >
            */
            booleanExpr(expr);
        }
    }