ex1(2).java

Thinking In Java 第四版练习题答案

// arrays/Ex1.java
// TIJ4 Chapter Arrays, Exercise 1, page 752
/* Create a method that takes an array of BerylliumSphere as an argument.
* Call the method, creating the argument dynamically. Demonstrate that 
* ordinary aggregate array intitialization doesn't work in this case.
* Discover the only situations where ordinary aggregate array initialization
* works, and where dynamic aggregate array initialization is redundant.
*/
import java.util.*;
import static net.mindview.util.Print.*;
class A { public String toString() { return "A Object"; } }

public class Ex1 {
	// Method that takes BerylliumSphere[] as argument
	// and prints the argument as array elements and as List or String:
	public static void test(BerylliumSphere[] a) { 
		print(Arrays.asList(a)); 		
	}
	// Generic version:
	public static <T> void test(T[] t) {
		print(Arrays.asList(t));
	}
	// int version:
	public static void test(int[] ia) { 
		print(Arrays.toString(ia));				
	}
		
	public static void main(String[] args) {
		print("For objects, e.g., BerylliumSpheres:");
		// Array is created and initialized (aggregate initialization)
		// but all elements are null, not BerylliumSpheres:
		test(new BerylliumSphere[3]);
		// Dynamic aggregate initialization works;
		// elements are now BerylliumSpheres:
		test(new BerylliumSphere[]{ 
			new BerylliumSphere(), new BerylliumSphere() });
		// Aggregate initialization this way works:
		BerylliumSphere[] a = { new BerylliumSphere(), 
			new BerylliumSphere(), new BerylliumSphere() };
		test(a);
		// Elements initialized to null:
		BerylliumSphere[] bsa = new BerylliumSphere[2];		
		test(bsa);
		bsa = a; 
		test(bsa);
		print("-------------");	
		print("For primitives, e.g., int:");
		// Dynamic aggregate initialization works:
		test(new int[]{ new Integer(0), new Integer(0) });
		// But may be considered redundant, since
		// aggregate initialization works:
		// elements initialized to zero (not null):
		test(new int[2]);
		// Ordinary aggregate initialization this way also works:
		int[] ia = { 1, 2, 3, };
		test(ia);
	}
}