我有2个以下的课程：
public class Counter{
private int a;
public Counter(String id){
a=0;
}
public void increment(){
a++;
}
public int tally(){
return a;
}
public String toString(){
return "Counter is - " + a;
}
}
..和
/******************************************************************************
* Compilation: javac StdOut.java
* Execution: java StdOut
* Dependencies: none
*
* Writes data of various types to standard output.
*
******************************************************************************/
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
import java.util.Locale;
/**
* This class provides methods for printing strings and numbers to standard output.
* <p>
* <b>Getting started.</b>
* To use this class, you must have {@code StdOut.class} in your
* Java classpath. If you used our autoinstaller, you should be all set.
* Otherwise, download
* <a href = "http://introcs.cs.princeton.edu/java/stdlib/StdOut.java">StdOut.java</a>
* and put a copy in your working directory.
* <p>
* Here is an example program that uses {@code StdOut}:
* <pre>
* public class TestStdOut {
* public static void main(String[] args) {
* int a = 17;
* int b = 23;
* int sum = a + b;
* StdOut.println("Hello, World");
* StdOut.printf("%d + %d = %d\n", a, b, sum);
* }
* }
* </pre>
* <p>
* <b>Differences with System.out.</b>
* The behavior of {@code StdOut} is similar to that of {@link System#out},
* but there are a few subtle differences:
* <ul>
* <li> {@code StdOut} coerces the character-set encoding to UTF-8,
* which is a standard character encoding for Unicode.
* <li> {@code StdOut} coerces the locale to {@link Locale#US},
* for consistency with {@link StdIn}, {@link Double#parseDouble(String)},
* and floating-point literals.
* <li> {@code StdOut} <em>flushes</em> standard output after each call to
* {@code print()} so that text will appear immediately in the terminal.
* </ul>
* <p>
* <b>Reference.</b>
* For additional documentation,
* see <a href="http://introcs.cs.princeton.edu/15inout">Section 1.5</a> of
* <em>Computer Science: An Interdisciplinary Approach</em>
* by Robert Sedgewick and Kevin Wayne.
*
* @author Robert Sedgewick
* @author Kevin Wayne
*/
public final class StdOut {
// force Unicode UTF-8 encoding; otherwise it's system dependent
private static final String CHARSET_NAME = "UTF-8";
// assume language = English, country = US for consistency with StdIn
private static final Locale LOCALE = Locale.US;
// send output here
private static PrintWriter out;
// this is called before invoking any methods
static {
try {
out = new PrintWriter(new OutputStreamWriter(System.out, CHARSET_NAME), true);
}
catch (UnsupportedEncodingException e) {
System.out.println(e);
}
}
// don't instantiate
private StdOut() { }
/**
* Closes standard output.
*/
public static void close() {
out.close();
}
/**
* Terminates the current line by printing the line-separator string.
*/
public static void println() {
out.println();
}
/**
* Prints an object to this output stream and then terminates the line.
*
* @param x the object to print
*/
public static void println(Object x) {
out.println(x);
}
/**
* Prints a boolean to standard output and then terminates the line.
*
* @param x the boolean to print
*/
public static void println(boolean x) {
out.println(x);
}
/**
* Prints a character to standard output and then terminates the line.
*
* @param x the character to print
*/
public static void println(char x) {
out.println(x);
}
/**
* Prints a double to standard output and then terminates the line.
*
* @param x the double to print
*/
public static void println(double x) {
out.println(x);
}
/**
* Prints an integer to standard output and then terminates the line.
*
* @param x the integer to print
*/
public static void println(float x) {
out.println(x);
}
/**
* Prints an integer to standard output and then terminates the line.
*
* @param x the integer to print
*/
public static void println(int x) {
out.println(x);
}
/**
* Prints a long to standard output and then terminates the line.
*
* @param x the long to print
*/
public static void println(long x) {
out.println(x);
}
/**
* Prints a short integer to standard output and then terminates the line.
*
* @param x the short to print
*/
public static void println(short x) {
out.println(x);
}
/**
* Prints a byte to standard output and then terminates the line.
* <p>
* To write binary data, see {@link BinaryStdOut}.
*
* @param x the byte to print
*/
public static void println(byte x) {
out.println(x);
}
/**
* Flushes standard output.
*/
public static void print() {
out.flush();
}
/**
* Prints an object to standard output and flushes standard output.
*
* @param x the object to print
*/
public static void print(Object x) {
out.print(x);
out.flush();
}
/**
* Prints a boolean to standard output and flushes standard output.
*
* @param x the boolean to print
*/
public static void print(boolean x) {
out.print(x);
out.flush();
}
/**
* Prints a character to standard output and flushes standard output.
*
* @param x the character to print
*/
public static void print(char x) {
out.print(x);
out.flush();
}
/**
* Prints a double to standard output and flushes standard output.
*
* @param x the double to print
*/
public static void print(double x) {
out.print(x);
out.flush();
}
/**
* Prints a float to standard output and flushes standard output.
*
* @param x the float to print
*/
public static void print(float x) {
out.print(x);
out.flush();
}
/**
* Prints an integer to standard output and flushes standard output.
*
* @param x the integer to print
*/
public static void print(int x) {
out.print(x);
out.flush();
}
/**
* Prints a long integer to standard output and flushes standard output.
*
* @param x the long integer to print
*/
public static void print(long x) {
out.print(x);
out.flush();
}
/**
* Prints a short integer to standard output and flushes standard output.
*
* @param x the short integer to print
*/
public static void print(short x) {
out.print(x);
out.flush();
}
/**
* Prints a byte to standard output and flushes standard output.
*
* @param x the byte to print
*/
public static void print(byte x) {
out.print(x);
out.flush();
}
/**
* Prints a formatted string to standard output, using the specified format
* string and arguments, and then flushes standard output.
*
*
* @param format the <a href = "http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html#syntax">format string</a>
* @param args the arguments accompanying the format string
*/
public static void printf(String format, Object... args) {
out.printf(LOCALE, format, args);
out.flush();
}
/**
* Prints a formatted string to standard output, using the locale and
* the specified format string and arguments; then flushes standard output.
*
* @param locale the locale
* @param format the <a href = "http://docs.oracle.com/javase/7/docs/api/java/util/Formatter.html#syntax">format string</a>
* @param args the arguments accompanying the format string
*/
public static void printf(Locale locale, String format, Object... args) {
out.printf(locale, format, args);
out.flush();
}
/**
* Unit tests some of the methods in {@code StdOut}.
*
* @param args the command-line arguments
*/
public static void main(String[] args) {
// write to stdout
StdOut.println("Test");
StdOut.println(17);
StdOut.println(true);
StdOut.printf("%.6f\n", 1.0/7.0);
}
}
在此之后我写了一个客户端类：
public class ObjectTest{
public static void main(String[] args){
Counter C1 = new Counter("Ones");
C1.toString(); //Does not print anything
StdOut.println(C1); //Output is 'Counter is - 0'
}
}
下面;
C1.toString();
不会打印任何内容，但
StdOut.println(C1); //Output is 'Counter is - 0'
打印'计数器为 - 0'。
请解释一下吗？
注意：这个问题是指罗伯特和凯文韦恩的Algoithms书，这有什么不同。
答案 0 :(得分：0)
您返回默认情况下不会打印任何内容的String，而是必须将其写入某处。
StdOut.println(C1);
与
相同StdOut.println(C1.toString());
这是你如何写回到stdout的值。
答案 1 :(得分：0)
C1.toString();返回String并且不会将其打印到控制台。
虽然StdOut.println(C1);（printWriter.println(Object);）会在内部调用toString() C1方法，但它会在toString()的帮助下将Unknown command line flag 'service-account-credential-file'. Did you mean: service_account_credential_file?
返回的字符串打印到控制台印刷作家。
答案 2 :(得分：0)
像其他人说的那样，你的toString方法正在返回String，但它没有被写出来。您可以使用System.out.println(C1)或System.out.println(C1.toString())。或者，您可以将toString方法设为空，并在方法内使用System.out.println。
例如：
public void toString()
{
System.out.println("Counter is - "+a);
}