com.google.common.base.Defaults.java Source code

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/*
 * Copyright (C) 2007 The Guava Authors
 *
 * Licensed 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 com.google.common.base;

import static com.google.common.base.Preconditions.checkNotNull;

import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

import javax.annotation.CheckReturnValue;
import javax.annotation.Nullable;

/**
 * This class provides default values for all Java types, as defined by the JLS.
 *
 * @author Ben Yu
 * @since 1.0
 */
@CheckReturnValue
public final class Defaults {
    private Defaults() {
    }

    private static final Map<Class<?>, Object> DEFAULTS;

    static {
        // Only add to this map via put(Map, Class<T>, T)
        Map<Class<?>, Object> map = new HashMap<Class<?>, Object>();
        put(map, boolean.class, false);
        put(map, char.class, '\0');
        put(map, byte.class, (byte) 0);
        put(map, short.class, (short) 0);
        put(map, int.class, 0);
        put(map, long.class, 0L);
        put(map, float.class, 0f);
        put(map, double.class, 0d);
        DEFAULTS = Collections.unmodifiableMap(map);
    }

    private static <T> void put(Map<Class<?>, Object> map, Class<T> type, T value) {
        map.put(type, value);
    }

    /**
     * Returns the default value of {@code type} as defined by JLS --- {@code 0} for numbers, {@code
     * false} for {@code boolean} and {@code '\0'} for {@code char}. For non-primitive types and
     * {@code void}, {@code null} is returned.
     */
    @Nullable
    public static <T> T defaultValue(Class<T> type) {
        // Primitives.wrap(type).cast(...) would avoid the warning, but we can't use that from here
        @SuppressWarnings("unchecked") // the put method enforces this key-value relationship
        T t = (T) DEFAULTS.get(checkNotNull(type));
        return t;
    }
}