如果你们用下面的方法来检查:

public static boolean isNumeric(String str) {
    NumberFormat formatter = NumberFormat.getInstance();
    ParsePosition pos = new ParsePosition(0);
    formatter.parse(str, pos);
    return str.length() == pos.getIndex();
}

然后输入非常长的字符串会发生什么，比如我调用这个方法:

System.out.println(isNumeric("94328948243242352525243242524243425452342343948923"));

结果是“真”，也是一个太大的数字! 如果你使用regex来检查，同样的事情也会发生! 所以我宁愿使用“解析”方法来检查，就像这样:

public static boolean isNumeric(String str) {
    try {
        int number = Integer.parseInt(str);
        return true;
    } catch (Exception e) {
        return false;
    }
}

结果就如我所料!

谷歌的Guava库提供了一个很好的辅助方法:你可以像使用Integer一样使用它。parseInt，但如果字符串没有解析为有效整数，它将返回null而不是抛出异常。注意，它返回的是Integer，而不是int，所以你必须将它转换/自动装箱回int。

例子:

String s1 = "22";
String s2 = "22.2";
Integer oInt1 = Ints.tryParse(s1);
Integer oInt2 = Ints.tryParse(s2);

int i1 = -1;
if (oInt1 != null) {
    i1 = oInt1.intValue();
}
int i2 = -1;
if (oInt2 != null) {
    i2 = oInt2.intValue();
}

System.out.println(i1);  // prints 22
System.out.println(i2);  // prints -1

但是，在当前发行版(Guava r11)中，它仍然被标记为@Beta。

我还没有对它进行基准测试。查看源代码，有一些开销来自大量的完整性检查，但最终他们使用Character.digit(string.charAt(idx))，类似，但略有不同，从@Ibrahim上面的答案。在它们的实现中没有异常处理开销。

这是我对这个问题的回答。

一个方便的方法，你可以使用任何类型的解析器来解析任何字符串:isParsable(对象解析器，字符串str)。解析器可以是Class或对象。这也将允许你使用你写的自定义解析器，应该适用于任何场景，例如:

isParsable(Integer.class, "11");
isParsable(Double.class, "11.11");
Object dateFormater = new java.text.SimpleDateFormat("yyyy.MM.dd G 'at' HH:mm:ss z");
isParsable(dateFormater, "2001.07.04 AD at 12:08:56 PDT");

下面是我的代码和方法描述。

import java.lang.reflect.*;

/**
 * METHOD: isParsable<p><p>
 * 
 * This method will look through the methods of the specified <code>from</code> parameter
 * looking for a public method name starting with "parse" which has only one String
 * parameter.<p>
 * 
 * The <code>parser</code> parameter can be a class or an instantiated object, eg:
 * <code>Integer.class</code> or <code>new Integer(1)</code>. If you use a
 * <code>Class</code> type then only static methods are considered.<p>
 * 
 * When looping through potential methods, it first looks at the <code>Class</code> associated
 * with the <code>parser</code> parameter, then looks through the methods of the parent's class
 * followed by subsequent ancestors, using the first method that matches the criteria specified
 * above.<p>
 * 
 * This method will hide any normal parse exceptions, but throws any exceptions due to
 * programmatic errors, eg: NullPointerExceptions, etc. If you specify a <code>parser</code>
 * parameter which has no matching parse methods, a NoSuchMethodException will be thrown
 * embedded within a RuntimeException.<p><p>
 * 
 * Example:<br>
 * <code>isParsable(Boolean.class, "true");<br>
 * isParsable(Integer.class, "11");<br>
 * isParsable(Double.class, "11.11");<br>
 * Object dateFormater = new java.text.SimpleDateFormat("yyyy.MM.dd G 'at' HH:mm:ss z");<br>
 * isParsable(dateFormater, "2001.07.04 AD at 12:08:56 PDT");<br></code>
 * <p>
 * 
 * @param parser    The Class type or instantiated Object to find a parse method in.
 * @param str   The String you want to parse
 * 
 * @return true if a parse method was found and completed without exception
 * @throws java.lang.NoSuchMethodException If no such method is accessible 
 */
public static boolean isParsable(Object parser, String str) {
    Class theClass = (parser instanceof Class? (Class)parser: parser.getClass());
    boolean staticOnly = (parser == theClass), foundAtLeastOne = false;
    Method[] methods = theClass.getMethods();

    // Loop over methods
    for (int index = 0; index < methods.length; index++) {
        Method method = methods[index];

        // If method starts with parse, is public and has one String parameter.
        // If the parser parameter was a Class, then also ensure the method is static. 
        if(method.getName().startsWith("parse") &&
            (!staticOnly || Modifier.isStatic(method.getModifiers())) &&
            Modifier.isPublic(method.getModifiers()) &&
            method.getGenericParameterTypes().length == 1 &&
            method.getGenericParameterTypes()[0] == String.class)
        {
            try {
                foundAtLeastOne = true;
                method.invoke(parser, str);
                return true; // Successfully parsed without exception
            } catch (Exception exception) {
                // If invoke problem, try a different method
                /*if(!(exception instanceof IllegalArgumentException) &&
                   !(exception instanceof IllegalAccessException) &&
                   !(exception instanceof InvocationTargetException))
                        continue; // Look for other parse methods*/

                // Parse method refuses to parse, look for another different method
                continue; // Look for other parse methods
            }
        }
    }

    // No more accessible parse method could be found.
    if(foundAtLeastOne) return false;
    else throw new RuntimeException(new NoSuchMethodException());
}


/**
 * METHOD: willParse<p><p>
 * 
 * A convienence method which calls the isParseable method, but does not throw any exceptions
 * which could be thrown through programatic errors.<p>
 * 
 * Use of {@link #isParseable(Object, String) isParseable} is recommended for use so programatic
 * errors can be caught in development, unless the value of the <code>parser</code> parameter is
 * unpredictable, or normal programtic exceptions should be ignored.<p>
 * 
 * See {@link #isParseable(Object, String) isParseable} for full description of method
 * usability.<p>
 * 
 * @param parser    The Class type or instantiated Object to find a parse method in.
 * @param str   The String you want to parse
 * 
 * @return true if a parse method was found and completed without exception
 * @see #isParseable(Object, String) for full description of method usability 
 */
public static boolean willParse(Object parser, String str) {
    try {
        return isParsable(parser, str);
    } catch(Throwable exception) {
        return false;
    }
}

// only int
public static boolean isNumber(int num) 
{
    return (num >= 48 && c <= 57); // 0 - 9
}

// is type of number including . - e E 
public static boolean isNumber(String s) 
{
    boolean isNumber = true;
    for(int i = 0; i < s.length() && isNumber; i++) 
    {
        char c = s.charAt(i);
        isNumber = isNumber & (
            (c >= '0' && c <= '9') || (c == '.') || (c == 'e') || (c == 'E') || (c == '')
        );
    }
    return isInteger;
}

// is type of number 
public static boolean isInteger(String s) 
{
    boolean isInteger = true;
    for(int i = 0; i < s.length() && isInteger; i++) 
    {
        char c = s.charAt(i);
        isInteger = isInteger & ((c >= '0' && c <= '9'));
    }
    return isInteger;
}

public static boolean isNumeric(String s) 
{
    try
    {
        Double.parseDouble(s);
        return true;
    }
    catch (Exception e) 
    {
        return false;
    }
}

这是我知道的最快的方法来检查字符串是否为数字:

public static boolean isNumber(String str){
  int i=0, len=str.length();
  boolean a=false,b=false,c=false, d=false;
  if(i<len && (str.charAt(i)=='+' || str.charAt(i)=='-')) i++;
  while( i<len && isDigit(str.charAt(i)) ){ i++; a=true; }
  if(i<len && (str.charAt(i)=='.')) i++;
  while( i<len && isDigit(str.charAt(i)) ){ i++; b=true; }
  if(i<len && (str.charAt(i)=='e' || str.charAt(i)=='E') && (a || b)){ i++; c=true; }
  if(i<len && (str.charAt(i)=='+' || str.charAt(i)=='-') && c) i++;
  while( i<len && isDigit(str.charAt(i)) ){ i++; d=true;}
  return i==len && (a||b) && (!c || (c && d));
}
static boolean isDigit(char c){
  return c>='0' && c<='9';
}

import java.util.Scanner;

public class TestDemo {
    public static void main(String[] args) {
        boolean flag = true;
        Scanner sc = new Scanner(System.in);
        System.out.println("Enter the String:");
        String str = sc.nextLine();

        for (int i = 0; i < str.length(); i++) {
            if(str.charAt(i) > 48 && str.charAt(i) < 58) {
                flag = false;
                break;
            }
        }

        if(flag == true) {
            System.out.println("String is a valid String.");
        } else {
            System.out.println("String contains number.");
        }
    }
}