How to set primary key auto increment in realm android

This is an old and already answered question, but I want to post another solution that I used. You can do like this:

Realm realm = Realm.getDefaultInstance();
// Realm transaction
realm.executeTransactionAsync(new Realm.Transaction() { 
    @Override
     public void execute(Realm bgRealm) {
         // Get the current max id in the users table
         Number maxId = bgRealm.where(users.class).max("id");
         // If there are no rows, currentId is null, so the next id must be 1
         // If currentId is not null, increment it by 1
         int nextId = (maxId == null) ? 1 : maxId.intValue() + 1;
         // User object created with the new Primary key
         users user = bgRealm.createObject(users.class, nextId);
         // Now you can update your object with your data. The object will be
         // automatically saved in the database when the execute method ends
         // ...
         // ... 
    }
}

Well, @PrimaryKey is only indicates that field is a key. But you set it yourself when creating object and copying it to Realm. Consider using UUID.random(), ot increment it manually. (as in answer from comment): Realm Auto Increament field example

In a transaction, you can always reliably access the current maximum ID, based on which you can increment that and use it as the basis for the next ID.

 realm.executeTransaction(new Realm.Transaction() { // must be in transaction for this to work
     @Override
     public void execute(Realm realm) {
         // increment index
         Number currentIdNum = realm.where(users.class).max(usersFields.ID);
         int nextId;
         if(currentIdNum == null) {
            nextId = 1;
         } else {
            nextId = currentIdNum.intValue() + 1;
         }
         users user = new users(); // unmanaged
         user.setId(nextId);
         //...
         realm.insertOrUpdate(user); // using insert API
     }
 }

there is an example that make a sequence to implement auto increment of primary key id:

https://raw.githubusercontent.com/505aaron/realm-migration-example/master/realm/sequencer.js

const sequencer = (realmInstance, schema, props) => new Promise((resolve, reject) => {
  let saved;

  try {
    realmInstance.write(() => {
      const obj = { ...props };

      if (typeof obj.id === 'undefined') {
        let seq = realmInstance.objects('Sequence').filtered(`name = "${schema}"`)[0];
        if (typeof seq === 'undefined') {
          seq = realmInstance.create('Sequence', { name: schema, value: 0 });
        }
        obj.id = seq.next();
      }
      saved = realmInstance.create(schema, obj, true);

      resolve({ ...saved });
    });
  } catch (e) {
    reject(e);
  }
});

export default sequencer;