Block: move difficulty target range check to AbstractBlockChain.add()

It doesn't make much sense in a getter. It should be checked when
adding a block to the chain, like all the other checks.

core/src/main/java/org/bitcoinj/core/AbstractBlockChain.java

@@ -34,6 +34,7 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import javax.annotation.Nullable;
+import java.math.BigInteger;
 import java.nio.ByteBuffer;
 import java.time.Instant;
 import java.time.temporal.ChronoUnit;
@@ -478,6 +479,10 @@ public abstract class AbstractBlockChain {
                 return false;
             }
 
+            BigInteger target = block.getDifficultyTargetAsInteger();
+            if (target.signum() <= 0 || target.compareTo(params.maxTarget) > 0)
+                throw new VerificationException("Difficulty target is out of range: " + target.toString());
+
             // If we want to verify transactions (ie we are running with full blocks), verify that block has transactions
             if (shouldVerifyTransactions() && block.getTransactions() == null)
                 throw new VerificationException("Got a block header while running in full-block mode");

core/src/main/java/org/bitcoinj/core/Block.java

@@ -452,14 +452,12 @@ public class Block extends Message {
 
     /**
      * Returns the difficulty target as a 256 bit value that can be compared to a SHA-256 hash. Inside a block the
-     * target is represented using a compact form. If this form decodes to a value that is out of bounds, an exception
-     * is thrown.
+     * target is represented using a compact form.
+     *
+     * @return difficulty target as 256-bit value
      */
-    public BigInteger getDifficultyTargetAsInteger() throws VerificationException {
-        BigInteger target = ByteUtils.decodeCompactBits(difficultyTarget);
-        if (target.signum() <= 0 || target.compareTo(params.maxTarget) > 0)
-            throw new VerificationException("Difficulty target is bad: " + target.toString());
-        return target;
+    public BigInteger getDifficultyTargetAsInteger() {
+        return ByteUtils.decodeCompactBits(difficultyTarget);
     }
 
     /** Returns true if the hash of the block is OK (lower than difficulty target). */

core/src/test/java/org/bitcoinj/core/BlockChainTest.java

@@ -234,7 +234,7 @@ public class BlockChainTest {
             // allowable difficulty.
             fail();
         } catch (VerificationException e) {
-            assertTrue(e.getMessage(), e.getCause().getMessage().contains("Difficulty target is bad"));
+            assertTrue(e.getMessage(), e.getCause().getMessage().contains("Difficulty target is out of range"));
         }
     }