多个活动，启动和传递数据

Question

I am attempting to create a list of users in an android app. The list is on the MainActivity, with a button which redirects to AddMember. AddMember will take input for one member. I am trying to pass the information back to MainActivity, however it fails before I even get to the AddMemberActivity, when testing. It stops after the button click on the MainActivity.

Trie implementation

The error message is:

java.lang.NullPointerException: Attempt to invoke virtual method 'char java.lang.String.charAt(int)' on a null object reference at attendance.Trie.get(Trie.java:117) at attendance.Trie.get(Trie.java:113) at attendance.MainActivity$1.onClick(MainActivity.java:49)

line 49 is

if (trie.get(name) != null) {

public class MainActivity extends AppCompatActivity {

private Button button;
private ListView list;
private Trie trie;
private ArrayAdapter<String> adapter;
private int count = 0;
private static final int REQUEST_CODE = 100;
private String name;

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    if(requestCode == REQUEST_CODE) {
        if(resultCode == RESULT_OK) {
           name = data.getStringExtra("name");
        }
    }
}

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    button = (Button) findViewById(R.id.addBtn);
    list = (ListView) findViewById(R.id.memberList);
    trie = new Trie();
    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(getApplicationContext(), AddMember.class);
            startActivityForResult(intent, REQUEST_CODE);
            adapter = new ArrayAdapter<String>(getApplicationContext(), R.layout.content_list_items,R.id.list_content, trie.traverse());
            list.setAdapter(adapter);
            //check that member was added
            if (trie.get(name) != null) {
                // <- look for item! 
                //made an alert to show member already exists
            } else {
                trie.put(name.toLowerCase(), count++);
                adapter.add(name);
            }
            adapter.notifyDataSetChanged();
        }
    });
}

}

public class AddMember extends AppCompatActivity {

//Array of options --> ArrayAdapter --> ListView

//ListView :{views, items.xml}

private static final int REQUEST_CODE = 100;

private Button button;
private EditText name;
private EditText phone;
private EditText email;
private ArrayAdapter<Member> adapter;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_upload_member);

    button = (Button) findViewById(R.id.uploadMember);
    name = (EditText) findViewById(R.id.edit_name);


    //add member button

    button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent();
            intent.putExtra("name", name.getText());
            setResult(RESULT_OK, intent);
            finish();
        }
    });
}

}

Answer 1

It is clear from crash log that you are attempting to get character from null string .

java.lang.String.charAt(int)' on a null object reference at attendance.Trie.get(Trie.java:117)

Reason : You are passing null value in get method. You never assign the name string and by default it is assigned with null. And you pass it with trie.get(name) .

Check your get method of Trie class and put a check of null. If name is not null then get the character from string otherwise return null.

@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {

    if(requestCode == REQUEST_CODE) {
        if(resultCode == RESULT_OK) {
           name = data.getStringExtra("name");
           if(adapter == null){
           adapter = new ArrayAdapter<String>(getApplicationContext(),   R.layout.content_list_items,R.id.list_content, trie.traverse());
            list.setAdapter(adapter);
            }
            //check that member was added
            if (trie.get(name) != null) {
                // <- look for item! 
                //made an alert to show member already exists
            } else {
                trie.put(name.toLowerCase(), count++);
                adapter.add(name);
            }
            adapter.notifyDataSetChanged();

        }
    }
}

Remove adapter code from onClick method

button.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            Intent intent = new Intent(getApplicationContext(), AddMember.class);
            startActivityForResult(intent, REQUEST_CODE);

    });