w on create :


        lastLocalization = (Button) findViewById(R.id.lastLocalization);
        lastLocalization.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View view){
                openLastLocalization();
            }
        });

        addDebtor = findViewById(R.id.addDebtor);
        addDebtor.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view){
                openAddDebtor();
            }
        });

        debtorsListButton = (Button) findViewById(R.id.debtorsList);
        debtorsListButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view){
                openDebtorsList();
            }
        });



niżej tam na dole:

    public void openAddDebtor(){
        Intent intent = new Intent(this, addDebtor.class);
        startActivity(intent);
    }

    public void openDebtorsList(){
        Intent intent = new Intent(this, debtorsList.class);
        startActivity(intent);
    }

    public void openLastLocalization(){
        Intent intent = new Intent(this, lastLocalization.class);
        startActivity(intent);