Add some convenience methods to Transaction for signing.

core/src/main/java/com/google/bitcoin/core/Transaction.java

@@ -740,6 +740,22 @@ public TransactionInput addSignedInput(TransactionOutPoint prevOut, Script scrip
         return addSignedInput(prevOut, scriptPubKey, sigKey, SigHash.ALL, false);
     }
 
+    /**
+     * Adds an input that points to the given output and contains a valid signature for it, calculated using the
+     * signing key.
+     */
+    public TransactionInput addSignedInput(TransactionOutput output, ECKey signingKey) {
+        return addSignedInput(output.getOutPointFor(), output.getScriptPubKey(), signingKey);
+    }
+
+    /**
+     * Adds an input that points to the given output and contains a valid signature for it, calculated using the
+     * signing key.
+     */
+    public TransactionInput addSignedInput(TransactionOutput output, ECKey signingKey, SigHash sigHash, boolean anyoneCanPay) {
+        return addSignedInput(output.getOutPointFor(), output.getScriptPubKey(), signingKey, sigHash, anyoneCanPay);
+    }
+
     /**
      * Removes all the inputs from this transaction.
      * Note that this also invalidates the length attribute

core/src/main/java/com/google/bitcoin/core/TransactionOutput.java

@@ -334,6 +334,14 @@ private void writeObject(ObjectOutputStream out) throws IOException {
         out.defaultWriteObject();
     }
 
+    /**
+     * Returns a new {@link TransactionOutPoint}, which is essentially a structure pointing to this output.
+     * Requires that this output is not detached.
+     */
+    public TransactionOutPoint getOutPointFor() {
+        return new TransactionOutPoint(params, getIndex(), getParentTransaction());
+    }
+
     /** Returns a copy of the output detached from its containing transaction, if need be. */
     public TransactionOutput duplicateDetached() {
         return new TransactionOutput(params, null, value, org.spongycastle.util.Arrays.clone(scriptBytes));