How to append new Array in JSON file stored in android

The structure you defined above is wrong, you cannot define two node with the same name user in a JSONObject.

If you want to store a list of user in to a file, you may store it like:

[
    {
        "name":"Jack",
        "age" :20
    },
    {
        "name":"Tom",
        "age" :20
    }
]

By the way, why not use SQLite database?

Gson simplifies this work. Add the gson dependency https://github.com/google/gson or retrofit's gson (both are same) implementation 'com.squareup.retrofit2:converter-gson:2.4.0'. From your details constructed the following pojo classes :

public class Example {
@SerializedName("user")
@Expose
private List<User> user = null;

public List<User> getUser() {
return user;
}

public void setUser(List<User> user) {
this.user = user;
}
}

public class User {
@SerializedName("identifier")
@Expose
private String identifier;
@SerializedName("name")
@Expose
private String name;
@SerializedName("contact")
@Expose
private List<Contact> contact = null;

public String getIdentifier() {
return identifier;
}

public void setIdentifier(String identifier) {
this.identifier = identifier;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

public List<Contact> getContact() {
return contact;
}

public void setContact(List<Contact> contact) {
this.contact = contact;
}
}

public class Contact {
@SerializedName("contact")
@Expose
private String contact;

public String getContact() {
return contact;
}

public void setContact(String contact) {
this.contact = contact;
}
}

You can use the following snippet to get details from string and append your dynamic data and update the file.

String lastObject = getStringFromFile(new FileInputStream(file));
Example newJsonToReplace = new Example();
List<User> usersList = new ArrayList();
try{
    Type typeOfObject = new TypeToken<Example>() {
    }.getType(); // type you will be getting from string
    Example example = new Gson().fromJson(lastObject, typeOfObject); // converting string to pojo using gson
    if(example != null){
        usersList.addAll(example.getUser());
    }
    usersList.add(yourYourToAdd); // add your user object
    newJsonToReplace.setUser(usersList);
    String updatedList = new Gson().toJson(newJsonToReplace, typeOfObject); // again converting to json to string
    writeJsonFile(updatedList); // will replace the existing content
}catch(Exception exception){
    Log.d("Json Error", e.toString());
}