Parse JSON in Java with a single 100-line method and no dependencies.

parseJson.java

import java.util.*;
import java.nio.CharBuffer;
import java.nio.BufferUnderflowException;

/**
 * A single-method JSON parser in 100 lines of Java.
 *
 * Returns a String, Boolean, Integer, Long, Double, null, a List of those, or a Map<String,Object> of those.
 * Intended to parse input which is expected to be valid. If you want 100% correct error handling, go elsewhere.
 * Does not exacly match the JSON parsing rules for numbers.
 *
 * To parse a String: parseJson(CharBuffer.wrap(string));
 */
public static Object parseJson(CharBuffer in) {
    int tell = in.position();
    try {
        char c;
        while ((c=in.get()) == ' ' || c == '\n' || c == '\r' || c == '\t') {
            tell++;
        }
        Object out;
        if (c == '{') {
            Map<String,Object> m = new LinkedHashMap<String,Object>();
            while ((c=in.get()) == ' ' || c == '\n' || c == '\r' || c == '\t');
            if (c != '}') {
                in.position(in.position() - 1);
                do {
                    String key = (String)parseJson(in);
                    while ((c=in.get()) == ' ' || c == '\n' || c == '\r' || c == '\t');
                    if (c == ':') {
                        m.put((String)key, parseJson(in));
                        tell = in.position();
                    } else {
                        throw new UnsupportedOperationException("expecting colon");
                    }
                    while ((c=in.get()) == ' ' || c == '\n' || c == '\r' || c == '\t');
                    if (c != ',' && c != '}') {
                        throw new UnsupportedOperationException("expecting comma or end-map");
                    }
                } while (c != '}');
            }
            out = m;
        } else if (c == '[') {
            List<Object> l = new ArrayList<Object>();
            while ((c=in.get()) == ' ' || c == '\n' || c == '\r' || c == '\t');
            if (c != ']') {
                in.position(in.position() - 1);
                do {
                    l.add(parseJson(in));
                    tell = in.position();
                    while ((c=in.get()) == ' ' || c == '\n' || c == '\r' || c == '\t');
                    if (c != ',' && c != ']') {
                        throw new UnsupportedOperationException("expecting comma or end-list");
                    }
                } while (c != ']');
            }
            out = l;
        } else if (c == '"') {
            StringBuilder sb = new StringBuilder();
            while ((c=in.get()) != '"') {
                if (c == '\\') {
                    c = in.get();
                    switch (c) {
                        case 'n': c = '\n'; break;
                        case 'r': c = '\r'; break;
                        case 't': c = '\t'; break;
                        case 'b': c = '\b'; break;
                        case 'f': c = '\f'; break;
                        case 'u': c = (char)Integer.parseInt(in.subSequence(0, 4).toString(), 16); in.position(in.position() + 4); break;
                    }
                }
                sb.append(c);
            }
            out = sb.toString();
        } else if (c == 't' && in.get() == 'r' && in.get() == 'u' && in.get() == 'e') {
            out = Boolean.TRUE;
        } else if (c == 'f' && in.get() == 'a' && in.get() == 'l' && in.get() == 's' && in.get() == 'e') {
            out = Boolean.FALSE;
        } else if (c == 'n' && in.get() == 'u' && in.get() == 'l' && in.get() == 'l') {
            out = null;
        } else if (c == '-' || (c >= '0' && c <= '9')) {
            StringBuilder sb = new StringBuilder();
            sb.append(c);
            while (in.hasRemaining()) {
                if ((c=in.get()) == '.' || c == 'e' || c == 'E' || (c >= '0' && c <= '9')) {
                    sb.append(c);
                } else {
                    in.position(in.position() - 1);
                    break;
                }
            }
            String s = sb.toString();
            try {
                Long l = Long.parseLong(s);
                if (l.longValue() == l.intValue()) {        // This can't be done with a ternary due to unboxing confusion
                    out = Integer.valueOf(l.intValue());
                } else {
                    out = l;
                }
            } catch (Exception e) {
                try {
                    out = Double.parseDouble(s);
                } catch (Exception e2) {
                    throw new UnsupportedOperationException("invalid number: " + s);
                }
            }
        } else {
            throw new UnsupportedOperationException("invalid " + (c >= ' ' && c < 0x80 ? "'" + ((char)c) + "'" : "U+" + Integer.toHexString(c)));
        }
        return out;
    } catch (BufferUnderflowException e) {
        throw (IllegalArgumentException)new IllegalArgumentException("Parse failed: unexpected EOF").initCause(e);
    } catch (ClassCastException e) {
        in.position(tell);
        throw new IllegalArgumentException("Parse failed at " + in.position() + ": expected string");
    } catch (UnsupportedOperationException e) {
        in.position(tell);
        throw new IllegalArgumentException("Parse failed at " + in.position() + ": " + e.getMessage());
    }
}