Explanation of adding unique key and how to add multiple data without updatingexisting data.
if database reference child is fixed string, new value will not add. just it will update the previous value. for example :
DatabaseReference myRef = FirebaseDatabase.getInstance().getReference();
String mText = // get data from editText or set your own custom data
now if I insert data like this:
myRef.child("abc").child("cba").setValue(mText);
every time I insert data it will update my previous data. It will not add new data. Because my reference is fixed here(myRef.child("abc").child("cba") // this one, which is always fixed).
Now change the the value of child "cba" to a random or dynamic value which will not fix. For example:
Random rand = new Random();
// Obtain a number between [0 - 49].
int n = rand.nextInt(50);
myRef.child("abc").child(String.valueOf(n)).setValue(mText);
In this case it will add a new value instead of updating. because this time reference is not fixed here. it is dynamic. push() method exactly do the same thing. it generates random key to maintain unique reference.
A reference link for CRUD operation in Realtime Database : Firebase CRUD
An Android project with CRUD operation example : Project Link
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
WifiManager wm = (WifiManager) getApplicationContext().getSystemService(WIFI_SERVICE);
String ip = Formatter.formatIpAddress(wm.getConnectionInfo().getIpAddress());// Java program to print BFS traversal from a given source vertex.
// BFS(int s) traverses vertices reachable from s.
import java.io.*;
import java.util.*;
// This class represents a directed graph using adjacency list
// representation
class Graph
{
private int V; // No. of vertices
private LinkedList<Integer> adj[]; //Adjacency Lists
// Constructor
Graph(int v)
{
V = v;
adj = new LinkedList[v];
for (int i=0; i<v; ++i)
adj[i] = new LinkedList();
}
// Function to add an edge into the graph
void addEdge(int v,int w)
{
adj[v].add(w);
}
// prints BFS traversal from a given source s
void BFS(int s)
{
// Mark all the vertices as not visited(By default
// set as false)
boolean visited[] = new boolean[V];
// Create a queue for BFS
LinkedList<Integer> queue = new LinkedList<Integer>();
// Mark the current node as visited and enqueue it
visited[s]=true;
queue.add(s);
while (queue.size() != 0)
{
// Dequeue a vertex from queue and print it
s = queue.poll();
System.out.print(s+" ");
// Get all adjacent vertices of the dequeued vertex s
// If a adjacent has not been visited, then mark it
// visited and enqueue it
Iterator<Integer> i = adj[s].listIterator();
while (i.hasNext())
{
int n = i.next();
if (!visited[n])
{
visited[n] = true;
queue.add(n);
}
}
}
}
// Driver method to
public static void main(String args[])
{
Graph g = new Graph(4);
g.addEdge(0, 1);
g.addEdge(0, 2);
g.addEdge(1, 2);
g.addEdge(2, 0);
g.addEdge(2, 3);
g.addEdge(3, 3);
System.out.println("Following is Breadth First Traversal "+
"(starting from vertex 2)");
g.BFS(2);
}
}
// This code is contributed by Aakash Hasija
public class NonActivityClass {
//can I access the variables and methods from here
MainActivity mainActivity;
public NonActivityClass(MainActivity activity) {
mainActivity = activity;
}
public void doSomething() {
String name = mainActivity.name;
}
}