Adding factoy methods to create DelayPolicy

Author: vsilaev

src/main/java/net/tascalate/concurrent/DelayPolicy.java

@@ -24,17 +24,34 @@
 
 import net.tascalate.concurrent.delays.BoundedMaxDelayPolicy;
 import net.tascalate.concurrent.delays.BoundedMinDelayPolicy;
+import net.tascalate.concurrent.delays.ExponentialDelayPolicy;
 import net.tascalate.concurrent.delays.FirstRetryNoDelayPolicy;
 import net.tascalate.concurrent.delays.FixedIntervalDelayPolicy;
 import net.tascalate.concurrent.delays.ProportionalRandomDelayPolicy;
 import net.tascalate.concurrent.delays.UniformRandomDelayPolicy;
 
 public interface DelayPolicy {
     public static final DelayPolicy DEFAULT = new FirstRetryNoDelayPolicy(new FixedIntervalDelayPolicy());
-    public static final DelayPolicy INVALID_DELAY = ctx -> -1;
+    public static final DelayPolicy INVALID = ctx -> -1;
 
     abstract public long delayMillis(RetryContext context);
 
+    public static DelayPolicy fixedInterval() {
+    	return new FixedIntervalDelayPolicy();
+    }
+    		
+    public static DelayPolicy fixedInterval(long intervalMillis) {
+    	return new FixedIntervalDelayPolicy(intervalMillis);
+    }
+    
+    public static DelayPolicy exponential(double multiplier) {
+    	return new ExponentialDelayPolicy(multiplier);
+    }
+    
+    public static DelayPolicy exponential(long initialDelayMillis, double multiplier) {
+    	return new ExponentialDelayPolicy(initialDelayMillis, multiplier);
+    }
+    
     default DelayPolicy withUniformJitter() {
         return new UniformRandomDelayPolicy(this);
     }

src/main/java/net/tascalate/concurrent/RetryPolicy.java

@@ -156,7 +156,7 @@ public RetryPolicy(int maxRetries, long defaultDelay) {
              Collections.emptySet(), Collections.emptySet(), 
              ctx -> true, ctx -> false, 
              DelayPolicy.DEFAULT,
-             DelayPolicy.INVALID_DELAY
+             DelayPolicy.INVALID
         );
     }
 

src/main/java/net/tascalate/concurrent/delays/ExponentialDelayPolicy.java

@@ -29,6 +29,10 @@ public class ExponentialDelayPolicy implements DelayPolicy {
     private final long initialDelayMillis;
     private final double multiplier;
 
+    public ExponentialDelayPolicy(double multiplier) {
+    	this(FixedIntervalDelayPolicy.DEFAULT_PERIOD_MILLIS, multiplier);
+    }
+    
     public ExponentialDelayPolicy(long initialDelayMillis, double multiplier) {
         if (initialDelayMillis <= 0) {
             throw new IllegalArgumentException("Initial delay must be positive but was: " + initialDelayMillis);