org.apache.calcite.linq4j.function.Parameter Maven / Gradle / Ivy
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to you under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.calcite.linq4j.function;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
/**
* Annotation that supplies metadata about a function parameter.
*
* A typical use is to derive names for the parameters of user-defined
* functions.
*
*
Here is an example:
*
*
* public static class MyLeftFunction {
* public String eval(
* @Parameter(name = "s") String s,
* @Parameter(name = "n", optional = true) Integer n) {
* return s.substring(0, n == null ? 1 : n);
* }
* }
*
* The first parameter is named "s" and is mandatory,
* and the second parameter is named "n" and is optional.
*
*
If this annotation is present, it supersedes information that might be
* available via
* {@code Executable.getParameters()} (JDK 1.8 and above).
*
*
If the annotation is not specified, parameters will be named "arg0",
* "arg1" et cetera, and will be mandatory.
*/
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.PARAMETER })
public @interface Parameter {
/** The name of the parameter.
*
*
The name is used when the function is called
* with parameter assignment, for example {@code foo(x => 1, y => 'a')}.
*/
String name();
/** Returns whether the parameter is optional.
*
*
An optional parameter does not need to be specified when you call the
* function.
*
*
If you call a function using positional parameter syntax, you can omit
* optional parameters on the trailing edge. For example, if you have a
* function
* {@code baz(int a, int b optional, int c, int d optional, int e optional)}
* then you can call {@code baz(a, b, c, d, e)}
* or {@code baz(a, b, c, d)}
* or {@code baz(a, b, c)}
* but not {@code baz(a, b)} because {@code c} is not optional.
*
*
If you call a function using parameter name assignment syntax, you can
* omit any parameter that has a default value. For example, you can call
* {@code baz(a => 1, e => 5, c => 3)}, omitting optional parameters {@code b}
* and {@code d}.
*
*
Currently, the default value used when a parameter is not specified
* is NULL, and therefore optional parameters must be nullable.
*/
boolean optional() default false;
}