Java SE: Effective Java Notes "Methods"_JAVA_编程开发_程序员俱乐部

Effective Java 2nd Edition, Chapter 7, Item 42

Be careful when using var args. Do not abuse var args in our custmoized methods.

1) Painful Arrays.asList for Primitive Types

class="java" name="code">package edu.xmu.guava.constant;

import static org.junit.Assert.assertEquals;

import java.util.Arrays;
import java.util.List;

import org.junit.Test;

import com.google.common.collect.Lists;
import com.google.common.primitives.Ints;

public class VarArgsTest {
    @Test
    public void varArgsTest() {
	int[] array = new int[] { 1, 2, 3, 4, 5 };
	List<int[]> list = Arrays.asList(array);
	assertEquals(1, list.size());

	list = Lists.newArrayList(array);
	assertEquals(1, list.size());
    }

    @Test
    public void varArgsTest2() {
	String[] array = new String[] { "1", "2", "3", "4", "5" };
	List<String> list = Arrays.asList(array);
	assertEquals(5, list.size());

	list = Lists.newArrayList(array);
	assertEquals(5, list.size());
    }

    @Test
    public void varArgsTest3() {
	int[] array = new int[] { 1, 2, 3, 4, 5 };
	List<Integer> list = Ints.asList(array);
	assertEquals(5, list.size());
    }
}

? ?1> The reason why Arrays.asList returns List<int[]> instead of List<Integer> is that:

? ? ? ? 1> Before JDK 1.4, Arrays.asList(new int[]{1, 2, 3, 4, 5}); will produce a compilation error:

asList(Object[]) in Arrays can't be applied to (int[])

? ? ? ? ? ? ?This is because the signature of Arrays.asList is:

public static <T> List<T> asList(T[] a);

? ? ? ? ? ? ?And our passed in array is not an Object[] array.

? ? ? ? 2> But for JDK 1.5 or above, Arrays.asList supports var args, and the signature becomes:

public static <T> List<T> asList(T... a);

? ? ? ? ? ? ?And our passed in array, instead of its contents, is considered as the first element in var args.

? ? ? ? ? ? ?Thus, the asList(int[]{...}) returns List<int[]>

? ? 2> How can we safely handle this?

? ? ? ? ?Use Ints.asList(int... backingArray) in guava.

? ? ? ? ?And guava provides handful asList implementation for every primitive types:

? ? ? ? ?1) Booleans.asList

? ? ? ? ?2) Bytes.asList

? ? ? ? ?3) Shorts.asList

? ? ? ? ?4) Ints.asList

? ? ? ? ?5) Longs.asList

? ? ? ? ?6) Floats.asList

? ? ? ? ?7) Doubles.asList
? ? ? ? ?8) Chars.asList

2) Performance Penalty

? ? 1> Everytime we invoke the method who accepts var args will cause a relocation and initiation of an array.

@Test
public void varArgsTest4() {
  String[] returnedStrs = dummyMethod("A", "B", "C");
  String[] returnedStrs2 = dummyMethod("A", "B", "C");
  String[] returnedStrs3 = dummyMethod("A", "B", "C");

  assertNotEquals(returnedStrs, returnedStrs2);
  assertNotEquals(returnedStrs, returnedStrs3);
}

private String[] dummyMethod(String... strs) {
  return strs;
}

? ? ? ?We can find out in example above that everytime we invoke the same dummyMethod, a totally new String[] will be returned.

? ? ? ?And it will affect our code's performance unintentionally.

? ? 2> How can we avoid this performance penalty and at the same time have the flexibility of var args.

? ? ? ?If we are pretty sure that this method will be invoked within 3 params at 95% confidence level, then we can make changes below to avoid performance penalty.

private void dummyMethod() {
}
private void dummyMethod(String var1) {
}
private void dummyMethod(String var1, String var2) {
}
private void dummyMethod(String var1, String var2, String var3) {
}
private void dummyMethod(String var1, String var2, String var3, String... rest) {
}

?

Reference Links:

1)?https://stackoverflow.com/questions/2607289/converting-array-to-list-in-java/2607327#2607327

2)?http://stackoverflow.com/questions/1073919/how-to-convert-int-into-listinteger-in-java