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// 源码基于 JDK 1.8
public final class String
    implements java.io.Serializable, Comparable<String>, CharSequence {
    // String 值的实际存储容器
    private final char value[];
    public String() {
        this.value = "".value;
    }
    public String(String original) {
        this.value = original.value;
        this.hash = original.hash;
    }
    // 忽略其他信息
}

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public static String doAdd() {
    String result = "";
    for (int i = 0; i < 10000; i++) {
        result += (" i:" + i);
    }
    return result;
}

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@Override
public synchronized StringBuffer append(CharSequence s) {
    toStringCache = null;
    super.append(s);
    return this;
}

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public static String doAppend() {
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < 10000; i++) {
        sb.append(" i:" + i);
    }
    return sb.toString();
}

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abstract class AbstractStringBuilder implements Appendable, CharSequence {
    char[] value;
    int count;
    @Override
    public AbstractStringBuilder append(CharSequence s, int start, int end) {
        if (s == null)
            s = "null";
        if ((start < 0) || (start > end) || (end > s.length()))
            throw new IndexOutOfBoundsException(
                "start " + start + ", end " + end + ", s.length() "
                + s.length());
        int len = end - start;
        ensureCapacityInternal(count + len);
        for (int i = start, j = count; i < end; i++, j++)
            value[j] = s.charAt(i);
        count += len;
        return this;
    }
    // 忽略其他信息...
}

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@Override
public String toString() {
    // Create a copy, don't share the array
    return new String(value, 0, count);
}

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/**
* Returns a canonical representation for the string object.
* <p>
* A pool of strings, initially empty, is maintained privately by the
* class {@code String}.
* <p>
* When the intern method is invoked, if the pool already contains a
* string equal to this {@code String} object as determined by
* the {@link #equals(Object)} method, then the string from the pool is
* returned. Otherwise, this {@code String} object is added to the
* pool and a reference to this {@code String} object is returned.
* <p>
* It follows that for any two strings {@code s} and {@code t},
* {@code s.intern() == t.intern()} is {@code true}
* if and only if {@code s.equals(t)} is {@code true}.
* <p>
* All literal strings and string-valued constant expressions are
* interned. String literals are defined in section 3.10.5 of the
* <cite>The Java&trade; Language Specification</cite>.
*
* @return  a string that has the same contents as this string, but is
*          guaranteed to be from a pool of unique strings.
*/
public native String intern();

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public class Location {
    private String city;
    private String region;
    private String countryCode;
    private double longitude;
    private double latitude;
}

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public class SharedLocation {

  private String city;
  private String region;
  private String countryCode;
}

public class Location {

  private SharedLocation sharedLocation;
  double longitude;
  double latitude;
}

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SharedLocation sharedLocation = new SharedLocation();
sharedLocation.setCity(messageInfo.getCity().intern());    
sharedLocation.setCountryCode(messageInfo.getRegion().intern());
sharedLocation.setRegion(messageInfo.getCountryCode().intern());

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String s1 = new String("Java中文社群").intern();
String s2 = new String("Java中文社群").intern();
System.out.println(s1 == s2);

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String badRegex = "^([hH][tT]{2}[pP]://|[hH][tT]{2}[pP][sS]://)(([A-Za-z0-9-~]+).)+([A-Za-z0-9-~\\\\/])+$";
String bugUrl = "http://www.apigo.com/dddp-web/pdf/download?request=6e7JGxxxxx4ILd-kExxxxxxxqJ4-CHLmqVnenXC692m74H38sdfdsazxcUmfcOH2fAfY1Vw__%5EDadIfJgiEf";
if (bugUrl.matches(badRegex)) {
    System.out.println("match!!");
} else {
    System.out.println("no match!!");
}

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text = "abbc";
regex = "ab{1,3}c";