#vulnerable code 
                                @Test
    public void testExplicitevaluate() {
        ExpressionFlipStrategy efs = new ExpressionFlipStrategy();
        Assert.assertTrue(efs.evaluate("D", ff4j.getStore(), null));
        Assert.assertTrue(efs.evaluate("TOTO", ff4j.getStore(), null));

        FlippingExecutionContext fex = new FlippingExecutionContext();
        fex.putString(ExpressionFlipStrategy.PARAM_EXPRESSION, "D");
        Assert.assertTrue(efs.evaluate("D", ff4j.getStore(), fex));

        fex.putString(ExpressionFlipStrategy.PARAM_EXPRESSION, "TOTO");
        Assert.assertFalse(efs.evaluate("D", ff4j.getStore(), fex));
    } 
                            #location 4 
                            #vulnerability type NULL_DEREFERENCE
#fixed code
                                @Test
    public void testExplicitevaluate() {
        ExpressionFlipStrategy efs = new ExpressionFlipStrategy();

        Assert.assertTrue(efs.evaluate("D", ff4j.getStore(), null));
        Assert.assertTrue(efs.evaluate("TOTO", ff4j.getStore(), null));

        FlippingExecutionContext fex = new FlippingExecutionContext();
        fex.putString(ExpressionFlipStrategy.PARAM_EXPRESSION, "D");
        Assert.assertTrue(efs.evaluate("D", ff4j.getStore(), fex));

        fex.putString(ExpressionFlipStrategy.PARAM_EXPRESSION, "TOTO");
        Assert.assertFalse(efs.evaluate("D", ff4j.getStore(), fex));
    }