f - Add Java example for chain::Filter

Author: jkczyz

docs/blockdata.md

@@ -146,11 +146,11 @@ optional type that implements `chain::Filter`:
 ```rust
 impl chain::Filter for Blockchain {
 	fn register_tx(&self, txid: &Txid, script_pubkey: &Script) {
-		//
+		// <insert code for you to watch for this transaction on-chain>
 	}
 
 	fn register_output(&self, outpoint: &OutPoint, script_pubkey: &Script) {
-		//
+		// <insert code for you to watch for this output on-chain>
 	}
 }
 ```
@@ -159,7 +159,17 @@ impl chain::Filter for Blockchain {
 <TabItem value="java">
 
 ```java
-// TODO
+Filter tx_filter = Filter.new_impl(new Filter.FilterInterface() {
+	@Override
+	public void register_tx(byte[] txid, byte[] script_pubkey) {
+		// <insert code for you to watch for this transaction on-chain>
+	}
+
+	@Override
+	void register_output(OutPoint outpoint, byte[] script_pubkey) {
+		// <insert code for you to watch for this output on-chain>
+	}
+});
 ```
 
 </TabItem>