bind: support casting of Java objects

Generate Cast functions that take a proxy for a Java class or interface,
and return a new proxy with the same reference. The Cast functions
panic if the underlying Java object is not an instance of the expected
type.

Change-Id: I08a5bf9a79139f0fac5dd102c7b028c8c989fc6d
Reviewed-on: https://go-review.googlesource.com/30095
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/ClassesTest.java

@@ -13,6 +13,7 @@ import java.util.Arrays;
 import java.util.Random;
 
 import go.javapkg.Javapkg;
+import go.javapkg.I;
 import go.javapkg.GoObject;
 import go.javapkg.GoRunnable;
 import go.javapkg.GoSubset;
@@ -127,4 +128,25 @@ public class ClassesTest extends InstrumentationTestCase {
 		Integer i = Javapkg.newJavaInteger();
 		assertEquals("new Integer(42)", 42, i.intValue());
 	}
+
+	private static class InterfaceRunnable implements I, Runnable {
+		@Override public void run() {
+		}
+	}
+
+	public void testCast() {
+		Runnable r1 = new GoRunnable();
+		Runnable r1c = Javapkg.castRunnable((Object)r1);
+		assertTrue("Casting Go object", r1c != null);
+		Runnable r2 = new Runnable() {
+			@Override public void run() {
+			}
+		};
+		Runnable r2c = Javapkg.castRunnable((Object)r2);
+		assertTrue("Casting Java object", r2c != null);
+		Runnable r3c = Javapkg.castInterface(new InterfaceRunnable());
+		assertTrue("Casting Go interface implementation", r3c != null);
+		Runnable r4c = Javapkg.castRunnable(new Object());
+		assertTrue("Invalid cast", r4c == null);
+	}
 }

bind/java/seq.h

@@ -59,5 +59,6 @@ extern void go_seq_pop_local_frame(JNIEnv *env);
 extern jclass go_seq_find_class(const char *name);
 extern jmethodID go_seq_get_static_method_id(jclass clazz, const char *name, const char *sig);
 extern jmethodID go_seq_get_method_id(jclass clazz, const char *name, const char *sig);
+extern int go_seq_isinstanceof(jint refnum, jclass clazz);
 
 #endif // __GO_SEQ_HDR__

bind/java/seq_android.c.support

@@ -382,3 +382,11 @@ void go_seq_release_byte_array(JNIEnv *env, jbyteArray arr, jbyte* ptr) {
 		(*env)->ReleaseByteArrayElements(env, arr, ptr, 0);
 	}
 }
+
+int go_seq_isinstanceof(jint refnum, jclass clazz) {
+	JNIEnv *env = go_seq_push_local_frame(0);
+	jobject obj = go_seq_from_refnum(env, refnum, NULL, NULL);
+	jboolean isinst = (*env)->IsInstanceOf(env, obj, clazz);
+	go_seq_pop_local_frame(env);
+	return isinst;
+}