public class DemoList extends AppCompatActivity {
    private ListView list_demo;
    private ArrayList<String> numberlist;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_demo_list);
        list_demo = (ListView) findViewById(R.id.list_demo);


        numberlist = new ArrayList<String>();
        getNumberList();
        int str1 = Integer.parseInt(Collections.min(numberlist));
        String str = Collections.max(numberlist).toString();
        //getMax(numberlist, str1);

        printInOrder(numberlist);
        Integer[] stringArray = numberlist.toArray(new Integer[0]);
        getMax(stringArray);
        ArrayAdapter<String> arrayAdapter =
                new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, numberlist);
        // Set The Adapter
        list_demo.setAdapter(arrayAdapter);
    }


    void getNumberList() {
        numberlist.add("1");
        numberlist.add("2");
        numberlist.add("7");
        numberlist.add("10");
        numberlist.add("15");
        numberlist.add("6");
        numberlist.add("3");
        numberlist.add("0");
        numberlist.add("25");
        numberlist.add("99");
        numberlist.add("70");
        numberlist.add("65");
        numberlist.add("1000");


    }

    public static void printInOrder(ArrayList data) {

    }

    public static int getMax(Integer[] inputArray) {
        int maxValue = inputArray[0];
        for (int i = 1; i < inputArray.length; i++) {
            if (inputArray[i] > maxValue) {
                maxValue = inputArray[i];
            }
        }
        return maxValue;
    }


}

Collections.max(yourArraylist);