You can simply use the new Java 8 Streams but you have to work with int.

The stream method of the utility class Arrays gives you an IntStream on which you can use the min method. You can also do max, sum, average,...

The getAsInt method is used to get the value from the OptionalInt

import java.util.Arrays;

public class Test {
    public static void main(String[] args){
        int[] tab = {12, 1, 21, 8};
        int min = Arrays.stream(tab).min().getAsInt();
        int max = Arrays.stream(tab).max().getAsInt();
        System.out.println("Min = " + min);
        System.out.println("Max = " + max)
    }

}

==UPDATE==

If execution time is important and you want to go through the data only once you can use the summaryStatistics() method like this

import java.util.Arrays;
import java.util.IntSummaryStatistics;

public class SOTest {
    public static void main(String[] args){
        int[] tab = {12, 1, 21, 8};
        IntSummaryStatistics stat = Arrays.stream(tab).summaryStatistics();
        int min = stat.getMin();
        int max = stat.getMax();
        System.out.println("Min = " + min);
        System.out.println("Max = " + max);
    }
}

This approach can give better performance than classical loop because the summaryStatistics method is a reduction operation and it allows parallelization.

Cleaner implementation of the accepted answer:

public static int findMax(int[] array) {

    // TODO: Take appropriate action if the array is empty.

    int max_value = Integer.MIN_VALUE;

    for ( int value : array ) {
        if ( value > max_value ) {
            max_value = value;
        }
    }

    return max_value;
}

you can try this -

class MaxNumber
{
    public static void main(String args[])
    {
        int[] a = new int[] { 10, 3, 50, 14, 7, 90};
        int max = a[0];
        for(int i = 1; i < a.length;i++)
        {
            if(a[i] > max)
            {
                max = a[i];
            }
        }

        System.out.println("Given Array is:");
        for(int i = 0; i < a.length;i++)
        {
            System.out.println(a[i]);
        }

        System.out.println("Max Number is:" + max);
    }
}