--- title: Changes to String in java (from 1.7.0_06) date: 2020-06-06 09:00:00 tags: - Algorithms categories: - notes keywords: - Algorithms - String - radix sort --- Before 1.7.0_06, `String` has 4 non static field: * char[] value * int[] offset * int count * int hash `Subing.substring` create a String by sharing the original String's internal `char[] value` and setting offset. This saves memory and makes `String.substring` run in a constant time($O(1)$). Meanwhile, this feature may cause **memory leak**[^1]. [http://hg.openjdk.java.net/jdk6/jdk6/jdk/file/8deef18bb749/src/share/classes/java/lang/String.java](http://hg.openjdk.java.net/jdk6/jdk6/jdk/file/8deef18bb749/src/share/classes/java/lang/String.java) ```Java public final class String implements java.io.Serializable, Comparable, CharSequence { /** The value is used for character storage. */ private final char value[]; /** The offset is the first index of the storage that is used. */ private final int offset; /** The count is the number of characters in the String. */ private final int count; /** Cache the hash code for the string */ private int hash; // Default to 0 // ... // Package private constructor which shares value array for speed. String(int offset, int count, char value[]) { this.value = value; this.offset = offset; this.count = count; } // ... /** * Returns a new string that is a substring of this string. The * substring begins at the specified beginIndex and * extends to the character at index endIndex - 1. * Thus the length of the substring is endIndex-beginIndex. *

* Examples: *

     * "hamburger".substring(4, 8) returns "urge"
     * "smiles".substring(1, 5) returns "mile"
     *

* * @param beginIndex the beginning index, inclusive. * @param endIndex the ending index, exclusive. * @return the specified substring. * @exception IndexOutOfBoundsException if the * beginIndex is negative, or * endIndex is larger than the length of * this String object, or * beginIndex is larger than * endIndex. */ public String substring(int beginIndex, int endIndex) { if (beginIndex < 0) { throw new StringIndexOutOfBoundsException(beginIndex); } if (endIndex > count) { throw new StringIndexOutOfBoundsException(endIndex); } if (beginIndex > endIndex) { throw new StringIndexOutOfBoundsException(endIndex - beginIndex); } return ((beginIndex == 0) && (endIndex == count)) ? this : new String(offset + beginIndex, endIndex - beginIndex, value); } // ... } ``` Since Java 1.7.0_06, `offset` and `count` fields were removed. `String.substring` makes new copies of `value`, which means we can forget about the memory leak but the runtime becomes $O(N)$ at the same time. [http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/String.java](http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/lang/String.java) ```Java public final class String implements java.io.Serializable, Comparable, CharSequence { /** The value is used for character storage. */ private final char value[]; /** Cache the hash code for the string */ private int hash; // Default to 0 // ... /** * Allocates a new {@code String} that contains characters from a subarray * of the character array argument. The {@code offset} argument is the * index of the first character of the subarray and the {@code count} * argument specifies the length of the subarray. The contents of the * subarray are copied; subsequent modification of the character array does * not affect the newly created string. * * @param value * Array that is the source of characters * * @param offset * The initial offset * * @param count * The length * * @throws IndexOutOfBoundsException * If the {@code offset} and {@code count} arguments index * characters outside the bounds of the {@code value} array */ public String(char value[], int offset, int count) { if (offset < 0) { throw new StringIndexOutOfBoundsException(offset); } if (count < 0) { throw new StringIndexOutOfBoundsException(count); } // Note: offset or count might be near -1>>>1. if (offset > value.length - count) { throw new StringIndexOutOfBoundsException(offset + count); } this.value = Arrays.copyOfRange(value, offset, offset+count); } // ... /** * Returns a string that is a substring of this string. The * substring begins at the specified {@code beginIndex} and * extends to the character at index {@code endIndex - 1}. * Thus the length of the substring is {@code endIndex-beginIndex}. *

* Examples: *

     * "hamburger".substring(4, 8) returns "urge"
     * "smiles".substring(1, 5) returns "mile"
     *