In the development of DNK, you will inexplicably encounter the problems of Java polymorphism, which subclasses and parent classes. Today, we will write a simple Demo for you to learn.

Java code is simple

  public static native String tt();

    public static String t(){
        List<String> l = new LinkedList();
        l.add("123");   //Simulating false data is not to get these data, in fact, the data is implemented in jni.
        l.add("456");
        return l.get(1);
    }

Among them, the local t() method in the class needs to be implemented in jni, that is, the t t() local method to implement the t() method.

jni Code:

jstring tt(JNIEnv *env, jclass type){

    LOGE(".....test java Polymorphism in jni Forms of expression.....");

    jobject l;   //Define ajobject

    jclass linkedList_clazz = findClass(env, "java/util/LinkedList");

    jobject linkedList_obj = newObject(env, linkedList_clazz);

    l=linkedList_obj;

    jclass list_clazz = findClass(env, "java/util/List");

    //Ljava/util/List;->add(Ljava/lang/Object;)Z

    jmethodID add_methodID = getMethodID(env, list_clazz, "add", "(Ljava/lang/Object;)Z");

    jmethodID get_methodID = getMethodID(env, list_clazz, "get", "(I)Ljava/lang/Object;");

    //
    env->CallBooleanMethod(l,add_methodID,env->NewStringUTF("Data 1"));

    env->CallBooleanMethod(l,add_methodID,env->NewStringUTF("Data 2"));

    //Ljava/util/List;->get(I)Ljava/lang/Object;

    jobject data = env->CallObjectMethod(l, get_methodID,1);  //Get the second data, and the collection starts at corner 0.

    return (jstring)data;
}

Code description:

java code

List l = new LinkedList();
List It's the parent class, and LinkedList yes List Subclass, create(new)The object process is actually Java The manifestation of polymorphism.

jni code

Make up some smali code <It's easy to understand if you understand Android decompilation>:

.method public static t()Ljava/lang/String;
    .registers 2

    .prologue
    .line 37
    new-instance v0, Ljava/util/LinkedList;

    invoke-direct {v0}, Ljava/util/LinkedList;-><init>()V

    .line 38
    .local v0, "l":Ljava/util/List;, "Ljava/util/List<Ljava/lang/String;>;"
    const-string v1, "123"

    invoke-interface {v0, v1}, Ljava/util/List;->add(Ljava/lang/Object;)Z

    .line 39
    const-string v1, "456"

    invoke-interface {v0, v1}, Ljava/util/List;->add(Ljava/lang/Object;)Z

    .line 40
    const/4 v1, 0x1

    invoke-interface {v0, v1}, Ljava/util/List;->get(I)Ljava/lang/Object;

    move-result-object v1

    check-cast v1, Ljava/lang/String;

    return-object v1
.end method

.method public static native tt()Ljava/lang/String;
.end method

Operation results:

12-25 20:34:13.450 8025-8025/com.tencent.mm.wvs E/=======capture=======:... test the manifestation of java polymorphism in jni...
12-25 20:34:13.451 8025-8025/com.tencent.mm.wvs E/fj: Get the second data of the list: Data 2