javaplugin lacks an useful method to create java interfaces.

very similar to 'new-class', this is my implementation:

@Command("new-interface")
public void newInterface(
@PipeIn final InputStream in,
@Option(required = false, help = "the package in which to build this Interface", description = "source package", type = PromptType.JAVA_PACKAGE, name = "package") final String pckg,
@Option(required = false, help = "the interface definition: surround with quotes", description = "interface definition") final String... def)
throws FileNotFoundException {

JavaSourceFacet java = project.getFacet(JavaSourceFacet.class);

JavaInterface jc = null;
if (def != null)

{ String classDef = Strings.join(Arrays.asList(def), " "); jc = JavaParser.parse(JavaInterface.class, classDef); }

else if (in != null)

{ jc = JavaParser.parse(JavaInterface.class, in); }

else

{ throw new RuntimeException("arguments required"); }

if (pckg != null)

{ jc.setPackage(pckg); }

if (!jc.hasSyntaxErrors())

{ java.saveJavaSource(jc); }

else {
writer.println(ShellColor.RED, "Syntax Errors:");
for (SyntaxError error : jc.getSyntaxErrors())

{ writer.println(error.toString()); }

writer.println();

if (prompt.promptBoolean(
"Your class has syntax errors, create anyway?", true))

{ java.saveJavaSource(jc); }

}
pickUp.fire(new PickupResource(java.getJavaResource(jc)));
}

i tested this and also the command 'new-method' works nicely:

@Test
public void testCreateJavaInterface() throws Exception {
getShell()
.execute(
"java new-interface --package org.jboss.forge.test.interfaces \"public interface TestingInterfaceCreation {}\"");
getShell().execute("build");
JavaInterface javaClass = (JavaInterface) getProject()
.getFacet(JavaSourceFacet.class)
.getJavaResource(
Packages.toFileSyntax("org.jboss.forge.test.interfaces.TestingInterfaceCreation"))
.getJavaSource();
assertNotNull(javaClass);
}

and

@Test
public void testCreateJavaMethodOnInterface() throws Exception {
getShell()
.execute(
"java new-interface --package org.jboss.forge.test.interfaces \"public interface TestingInterfaceCreation {}\"");
getShell().execute("java new-method \"public void testing();\"");
getShell().execute("ls");
getShell().execute("build");
JavaInterface javaClass = (JavaInterface) getProject()
.getFacet(JavaSourceFacet.class)
.getJavaResource(
Packages.toFileSyntax("org.jboss.forge.test.interfaces.TestingInterfaceCreation"))
.getJavaSource();
Method<JavaInterface> method = javaClass.getMethod("testing");
assertNotNull(method);
}