Cover all patterns of transactions.

PatternsOfTransactions.java

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.List;

/**
 * 
 * Cover all patterns of transactions.
 * 
 * @author kencoba
 *
 */
public class PatternsOfTransactions {
    public static void main(String[] args) {
        List<String> tranA = List.of("a.begin()","a.read(x)", "a.write(x)", "a.commit()");
        List<String> tranB = List.of("b.begin()","b.read(x)", "b.commit()");
        List<String> output = new ArrayList<String>();
        
        rec(tranA,tranB,output);
    }

    private static void rec(List<String> tranA, List<String> tranB, List<String> output) {
        if(tranA.isEmpty() & tranB.isEmpty()) {
            System.out.println(String.join("; ",output));
            return;
        }
        if (!tranA.isEmpty()) {
            String a = tranA.getFirst();
            rec(removeFirst(tranA),tranB,appendElements(output,a));
        }
        if (!tranB.isEmpty()) {
            String b = tranB.getFirst();
            rec(tranA,removeFirst(tranB),appendElements(output,b));
        }
    }
    
    @SafeVarargs
    static <T> List<T> appendElements(List<T> immutableList, T... elements) {
        List<T> tmpList = new ArrayList<>(immutableList);
        tmpList.addAll(Arrays.asList(elements));
        return Collections.unmodifiableList(tmpList);
    }
    
    static <T> List<T> removeFirst(List<T> immutableList) {
        List<T> tmpList = new ArrayList<>(immutableList);
        tmpList.removeFirst();
        return Collections.unmodifiableList(tmpList);
    }
}