Fix and Test String.equals(Object) Implementation in JPF (#491)

* adding a test for equals method

* Add test for equals method in JPF

output.txt

@@ -0,0 +1 @@
+After.

src/peers/gov/nasa/jpf/vm/JPF_java_lang_String.java

@@ -165,16 +165,16 @@ public boolean equals0___3C_3CI__Z (MJIEnv env, int clsObjRef, int charsRef1, in
 
   @MJI
   public boolean equals__Ljava_lang_Object_2__Z (MJIEnv env, int objRef, int argRef) {
-    if (argRef == MJIEnv.NULL) { 
-      return false; 
+    if (argRef == MJIEnv.NULL) {
+      return false;
     }
-
-    String s1 = env.getStringObject(objRef);
-    String s2 = env.getStringObject(argRef);
-
-    return s1.equals(s2);
+    if (env.isInstanceOf(argRef, "java.lang.String")) {
+      String s1 = env.getStringObject(objRef);
+      String s2 = env.getStringObject(argRef);
+      return s1.equals(s2);
+    }
+    return false;
   }
-
   @MJI
   public boolean equalsIgnoreCase__Ljava_lang_String_2__Z (MJIEnv env, int objref, int anotherString) {
     String thisString = env.getStringObject(objref);

src/tests/java11/StringEqualsTest.java

@@ -0,0 +1,15 @@
+package java11;
+
+import gov.nasa.jpf.util.test.TestJPF;
+import org.junit.Test;
+public class StringEqualsTest extends TestJPF {
+    @Test
+    public void testStringEqual_differentObj(){
+        if(verifyNoPropertyViolation()){
+            String word="a";
+            boolean actual=word.equals(new Object());
+            boolean expected =false;
+            assertEquals(expected,actual);
+        }
+    }
+}