Add default methods in Job and Step interfaces

Default values are consistent with AbstractJob
and AbstractStep.

Issue spring-projects#3924

spring-batch-core/src/main/java/org/springframework/batch/core/Job.java

@@ -15,6 +15,7 @@
  */
 package org.springframework.batch.core;
 
+import org.springframework.batch.core.job.DefaultJobParametersValidator;
 import org.springframework.lang.Nullable;
 
 /**
@@ -33,9 +34,11 @@ public interface Job {
 	/**
 	 * Flag to indicate if this job can be restarted, at least in principle.
 	 *
-	 * @return true if this job can be restarted after a failure
+	 * @return true if this job can be restarted after a failure. Defaults to {@code true}.
 	 */
-	boolean isRestartable();
+	default boolean isRestartable() {
+		return true;
+	}
 
 	/**
 	 * Run the {@link JobExecution} and update the meta information, such as status
@@ -52,19 +55,23 @@ public interface Job {
 	 * sequence, they can use this incrementer. The return value may be {@code null},
 	 * when this job does not have a natural sequence.
 	 *
-	 * @return in incrementer to be used for creating new parameters
+	 * @return an incrementer to be used for creating new parameters. Defaults to {@code null}.
 	 */
 	@Nullable
-	JobParametersIncrementer getJobParametersIncrementer();
+	default JobParametersIncrementer getJobParametersIncrementer() {
+		return null;
+	}
 
 	/**
 	 * A validator for the job parameters of a {@link JobExecution}. Clients of
 	 * a {@code Job} may need to validate the parameters for a launch or before or during
 	 * the execution.
 	 *
 	 * @return a validator that can be used to check parameter values (never
-	 * {@code null}).
+	 * {@code null}). Defaults to {@link DefaultJobParametersValidator}.
 	 */
-	JobParametersValidator getJobParametersValidator();
+	default JobParametersValidator getJobParametersValidator() {
+		return new DefaultJobParametersValidator();
+	}
 
 }

spring-batch-core/src/main/java/org/springframework/batch/core/Step.java

@@ -20,6 +20,7 @@
  * explicitly represent the configuration of a step by a developer but also the ability to execute the step.
  *
  * @author Dave Syer
+ * @author Mahmoud Ben Hassine
  *
  */
 public interface Step {
@@ -36,13 +37,19 @@ public interface Step {
 
 	/**
 	 * @return {@code true} if a step that is already marked as complete can be started again.
+	 * Defaults to {@code false}.
 	 */
-	boolean isAllowStartIfComplete();
+	default boolean isAllowStartIfComplete() {
+		return false;
+	}
 
 	/**
-	 * @return the number of times a job can be started with the same identifier.
+	 * @return the number of times a step can be (re)started for the same job instance.
+	 * Defaults to {@code Integer.MAX_VALUE}
 	 */
-	int getStartLimit();
+	default int getStartLimit() {
+		return Integer.MAX_VALUE;
+	}
 
 	/**
 	 * Process the step and assign progress and status meta information to the {@link StepExecution} provided. The